Added CHANGELOG item

CHANGELOG

@@ -16,6 +16,7 @@ v 8.7.0 (unreleased)
   - Fix avatar stretching by providing a cropping feature
   - API: Expose `subscribed` for issues and merge requests (Robert Schilling)
   - Allow SAML to handle external users based on user's information !3530
+  - Allow Omniauth providers to be marked as `external` !3657
   - Add endpoints to archive or unarchive a project !3372
   - Add links to CI setup documentation from project settings and builds pages
   - Handle nil descriptions in Slack issue messages (Stan Hu)