Enable release notes feature for Derek

This feature enables notes to be collected from all merged/closed
PRs since the last release. The notes are then set/saved on the
release body field.

.DEREK.yml

@@ -19,6 +19,7 @@ features:
  - comments
  - pr_description_required
  - hacktoberfest
+ - release_notes
 
 custom_messages:
   - name: slack