Merge branch 'doc-git-object-dedup' into 'master'

Document how Git object deduplication works in GitLab

See merge request gitlab-org/gitlab-ce!25770

doc/development/README.md

@@ -54,6 +54,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Prometheus metrics](prometheus_metrics.md)
 - [Guidelines for reusing abstractions](reusing_abstractions.md)
 - [DeclarativePolicy framework](policies.md)
+- [How Git object deduplication works in GitLab](git_object_deduplication.md)
 
 ## Performance guides