Merge branch 'update-github-doc-page' into 'master'

Update GitHub importer documentation  to reflect current importer state

## What does this MR do?

Update the Github importer documentation to reflect the current importer state.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/904

## Does this MR meet the acceptance criteria?

- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)


See merge request !5987

doc/workflow/importing/import_projects_from_github.md

 # Import your project from GitHub to GitLab
 
+Import your projects from GitHub to GitLab with minimal effort.
+
+## Overview
+
 >**Note:**
-In order to enable the GitHub import setting, you may also want to
-enable the [GitHub integration][gh-import] in your GitLab instance. This
-configuration is optional, you will be able import your GitHub repositories
-with a Personal Access Token.
+If you are an administrator you can enable the [GitHub integration][gh-import]
+in your GitLab instance sitewide. This configuration is optional, users will be
+able import their GitHub repositories with a [personal access token][gh-token].
 
-At its current state, GitHub importer can import:
+- At its current state, GitHub importer can import:
+  - the repository description (GitLab 7.7+)
+  - the Git repository data (GitLab 7.7+)
+  - the issues (GitLab 7.7+)
+  - the pull requests (GitLab 8.4+)
+  - the wiki pages (GitLab 8.4+)
+  - the milestones (GitLab 8.7+)
+  - the labels (GitLab 8.7+)
+  - the release note descriptions (GitLab 8.12+)
+- References to pull requests and issues are preserved (GitLab 8.7+)
+- Repository public access is retained. If a repository is private in GitHub
+  it will be created as private in GitLab as well.
 
-- the repository description (introduced in GitLab 7.7)
-- the git repository data (introduced in GitLab 7.7)
-- the issues (introduced in GitLab 7.7)
-- the pull requests (introduced in GitLab 8.4)
-- the wiki pages (introduced in GitLab 8.4)
-- the milestones (introduced in GitLab 8.7)
-- the labels (introduced in GitLab 8.7)
-- the release note descriptions (introduced in GitLab 8.12)
+## How it works
 
-With GitLab 8.7+, references to pull requests and issues are preserved.
+When issues/pull requests are being imported, the GitHub importer tries to find
+the GitHub author/assignee in GitLab's database using the GitHub ID. For this
+to work, the GitHub author/assignee should have signed in beforehand in GitLab
+and [**associated their GitHub account**][social sign-in]. If the user is not
+found in GitLab's database, the project creator (most of the times the current
+user that started the import process) is set as the author, but a reference on
+the issue about the original GitHub author is kept.
 
-The importer page is visible when you [create a new project][new-project].
-Click on the **GitHub** link and, if you are logged in via the GitHub
-integration, you will be redirected to GitHub for permission to access your
-projects. After accepting, you'll be automatically redirected to the importer.
+The importer will create any new namespaces (groups) if they don't exist or in
+the case the namespace is taken, the repository will be imported under the user's
+namespace that started the import process.
 
-If you are not using the GitHub integration, you can still perform a one-off
-authorization with GitHub to access your projects.
+## Importing your GitHub repositories
 
-Alternatively, you can also enter a GitHub Personal Access Token. Once you enter
-your token, you'll be taken to the importer.
+The importer page is visible when you create a new project.
 
 ![New project page on GitLab](img/import_projects_from_github_new_project_page.png)
 
----
+Click on the **GitHub** link and the import authorization process will start.
+There are two ways to authorize access to your GitHub repositories:
 
-While at the GitHub importer page, you can see the import statuses of your
-GitHub projects. Those that are being imported will show a _started_ status,
-those already imported will be green, whereas those that are not yet imported
-have an **Import** button on the right side of the table. If you want, you can
-import all your GitHub projects in one go by hitting **Import all projects**
-in the upper left corner.
+1. [Using the GitHub integration][gh-integration] (if it's enabled by your
+   GitLab administrator). This is the preferred way as it's possible to
+   preserve the GitHub authors/assignees. Read more in the [How it works](#how-it-works)
+   section.
+1. [Using a personal access token][gh-token] provided by GitHub.
 
-![GitHub importer page](img/import_projects_from_github_importer.png)
+![Select authentication method](img/import_projects_from_github_select_auth_method.png)
+
+### Authorize access to your repositories using the GitHub integration
 
----
+If the [GitHub integration][gh-import] is enabled by your GitLab administrator,
+you can use it instead of the personal access token.
+
+1. First you may want to connect your GitHub account to GitLab in order for
+   the username mapping to be correct. Follow the [social sign-in] documentation
+   on how to do so.
+1. Once you connect GitHub, click the **List your GitHub repositories** button
+   and you will be redirected to GitHub for permission to access your projects.
+1. After accepting, you'll be automatically redirected to the importer.
+
+You can now go on and [select which repositories to import](#select-which-repositories-to-import).
+
+### Authorize access to your repositories using a personal access token
+
+>**Note:**
+For a proper author/assignee mapping for issues and pull requests, the
+[GitHub integration][gh-integration] should be used instead of the
+[personal access token][gh-token]. If the GitHub integration is enabled by your
+GitLab administrator, it should be the preferred method to import your repositories.
+Read more in the [How it works](#how-it-works) section.
 
-The importer will create any new namespaces if they don't exist or in the
-case the namespace is taken, the project will be imported on the user's
-namespace.
+If you are not using the GitHub integration, you can still perform a one-off
+authorization with GitHub to grant GitLab access your repositories:
+
+1. Go to <https://github.com/settings/tokens/new>.
+1. Enter a token description.
+1. Check the `repo` scope.
+1. Click **Generate token**.
+1. Copy the token hash.
+1. Go back to GitLab and provide the token to the GitHub importer.
+1. Hit the **List your GitHub repositories** button and wait while GitLab reads
+   your repositories' information. Once done, you'll be taken to the importer
+   page to select the repositories to import.
+
+### Select which repositories to import
+
+After you've authorized access to your GitHub repositories, you will be
+redirected to the GitHub importer page.
+
+From there, you can see the import statuses of your GitHub repositories.
+
+- Those that are being imported will show a _started_ status,
+- those already successfully imported will be green with a _done_ status,
+- whereas those that are not yet imported will have an **Import** button on the
+  right side of the table.
+
+If you want, you can import all your GitHub projects in one go by hitting
+**Import all projects** in the upper left corner.
+
+![GitHub importer page](img/import_projects_from_github_importer.png)
 
 [gh-import]: ../../integration/github.md "GitHub integration"
-[ee-gh]: http://docs.gitlab.com/ee/integration/github.html "GitHub integration for GitLab EE"
 [new-project]: ../../gitlab-basics/create-project.md "How to create a new project in GitLab"
+[gh-integration]: #authorize-access-to-your-repositories-using-the-github-integration
+[gh-token]: #authorize-access-to-your-repositories-using-a-personal-access-token
+[social sign-in]: ../../profile/account/social_sign_in.md