Document the ee-files-location-check CI job

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

doc/development/ee_features.md

@@ -33,6 +33,40 @@ rest of the code should be as close to the CE files as possible.
 
 [single code base]: https://gitlab.com/gitlab-org/gitlab-ee/issues/2952#note_41016454
 
+### Detection of EE-only files
+
+For each commit (except on `master`), the `ee-files-location-check` CI job tries
+to detect if there are any new files that are EE-only. If any file is detected,
+the job fails with an explanation of why and what to do to make it pass.
+
+Basically, the fix is simple: `git mv <file> ee/<file>`.
+
+#### How to name your branches?
+
+For any EE branch, the job will try to detect its CE counterpart by removing any
+`ee-` prefix or `-ee` suffix from the EE branch name, and matching the last
+branch that contains it.
+
+For instance, from the EE branch `new-shiny-feature-ee` (or
+`ee-new-shiny-feature`), the job would find the corresponding CE branches:
+
+- `new-shiny-feature`
+- `ce-new-shiny-feature`
+- `new-shiny-feature-ce`
+- `my-super-new-shiny-feature-in-ce`
+
+#### Whitelist some EE-only files that cannot be moved to `ee/`
+
+The `ee-files-location-check` CI job provides a whitelist of files or folders
+that cannot or should not be moved to `ee/`. Feel free to open an issue to
+discuss adding a new file/folder to this whitelist.
+
+For instance, it was decided that moving EE-only files from `qa/` to `ee/qa/`
+would make it difficult to build the `gitLab-{ce,ee}-qa` Docker images and it
+was [not worth the complexity].
+
+[not worth the complexity]: https://gitlab.com/gitlab-org/gitlab-ee/issues/4997#note_59764702
+
 ### EE-only features
 
 If the feature being developed is not present in any form in CE, we don't
@@ -52,6 +86,11 @@ is applied not only to models. Here's a list of other examples:
 - `ee/app/validators/foo_attr_validator.rb`
 - `ee/app/workers/foo_worker.rb`
 
+This works because for every path that are present in CE's eager-load/auto-load
+paths, we add the same `ee/`-prepended path in [`config/application.rb`].
+
+[`config/application.rb`]: https://gitlab.com/gitlab-org/gitlab-ee/blob/d278b76d6600a0e27d8019a0be27971ba23ab640/config/application.rb#L41-51
+
 ### EE features based on CE features
 
 For features that build on existing CE features, write a module in the