Add code comments development guide

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/development/README.md

@@ -108,6 +108,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Database Debugging and Troubleshooting](database_debugging.md)
 - [Query Count Limits](query_count_limits.md)
 - [Database helper modules](database_helpers.md)
+- [Code comments](code_comments.md)
 
 ## Integration guides
 

doc/development/code_comments.md

+# Code comments
+
+Whenever you add comment to the code that is expected to be addressed at any time 
+in future, please create a technical debt issue for it. Then put a link to it 
+to the code comment you've created. This will allow other developers to quickly
+check if a comment is still relevant and what needs to be done to address it.
+
+Examples: 
+
+```rb
+# Deprecated scope until code_owner column has been migrated to rule_type.
+# To be removed with https://gitlab.com/gitlab-org/gitlab-ee/issues/11834.
+scope :code_owner, -> { where(code_owner: true).or(where(rule_type: :code_owner)) }
+```