Add doc file for dev rake tasks

doc/development/README.md

-+ [Architecture](architecture.md)
-+ [Shell commands](shell_commands.md)
+## Development 
+
++ [Architecture](architecture.md) of GitLab
++ [Shell commands](shell_commands.md) in the GitLab codebase
++ [Rake tasks](rake_tasks.md) for development

doc/development/rake_tasks.md

+# Rake tasks for developers
+
+## Setup db with developer seeds:
+
+Note that if your db user does not have advanced privilegies you must create db manually before run this command 
+
+```
+bundle exec rake setup
+```
+
+## Run tests
+
+This runs all test suite present in GitLab 
+
+```
+bundle exec rake test
+```
+
+## Generate searchable docs for source code
+
+You can find results under `doc/code` directory
+
+```
+bundle exec rake gitlab:generate_docs
+```