Merge branch 'missing_update_links' into 'master'

Missing omnibus update links in help/update

I took the liberty and refactored it a little. Open issue https://gitlab.com/gitlab-org/omnibus-gitlab/issues/269

Here is a screen

![gitlab_update_page](https://gitlab.com/uploads/axil/gitlab-ce/0395154f23/gitlab_update_page.png)

See merge request !202

doc/update/README.md

-- [The individual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update)
-- [Upgrader](upgrader.md)
-- [Patch versions](patch_versions.md)
-- [MySQL to PostgreSQL](mysql_to_postgresql.md)
+Depending on the installation method and your GitLab version, there are multiple update guides. Choose one that fits your needs.
+
+## Omnibus Packages
+
+- [Omnibus update guide](https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/update.md) contains the steps needed to update a GitLab [package](https://about.gitlab.com/downloads/).
+
+## Manual Installation
+
+- [The individual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update) are for those who have installed GitLab manually.
+- [The CE to EE update guides](https://gitlab.com/subscribers/gitlab-ee/tree/master/doc/update) are for subscribers of the Enterprise Edition only. The steps are very similar to a version upgrade: stop the server, get the code, update config files for the new functionality, install libs and do migrations, update the init script, start the application and check the application status.
+- [Upgrader](upgrader.md) is an automatic ruby script that performs the update for manual installations.
+- [Patch versions](patch_versions.md) guide includes the steps needed for a patch version, eg. 6.2.0 to 6.2.1.
+
+## Miscellaneous
+
+- [MySQL to PostgreSQL](mysql_to_postgresql.md) guides you through migrating your database from MySQL to PostrgreSQL.