Add the documentation for ee feature steps.

doc/development/ee_features.md

+# Guidelines for implementing Enterprise Edition feature
+
+## Write the code
+
+Implement the wanted feature.
+
+## Write the tests
+
+Implemented feature needs to have the full test coverage. For now, exception is the code that needs to query a LDAP server.
+
+## Write the documentation
+
+Any feature needs to be well documented. Add the documentation to `/doc` directory, describe the main use of the newly implemented feature and, if applicable, add screenshots.
+
+## Submit the MR to `about.gitlab.com`
+
+Submit the MR to [about.gitlab.com site repository](https://gitlab.com/gitlab-com/www-gitlab-com) to add the new feature to the [EE feature comparison page](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/gitlab-ee/index.html)