Add a link to the "Guidelines for implementing Enterprise Edition feature" page

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/README.md

@@ -23,6 +23,7 @@
 
 - [Generate a changelog entry with `bin/changelog`](changelog.md)
 - [Limit conflicts with EE when developing on CE](limit_ee_conflicts.md)
+- [Guidelines for implementing Enterprise Edition feature](ee_features.md)
 - [Code review guidelines](code_review.md) for reviewing code and having code reviewed.
 - [Merge request performance guidelines](merge_request_performance_guidelines.md)
   for ensuring merge requests do not negatively impact GitLab performance