Merge branch 'docs-enable-feature-flag' into 'master'

Add instructions on how to enable a feature flag

Closes gitlab-org/release/framework#23

See merge request gitlab-org/gitlab-ce!22518

doc/development/feature_flags.md

@@ -112,3 +112,8 @@ feature flag. You can stub a feature flag as follows:
 ```ruby
 stub_feature_flags(my_feature_flag: false)
 ```
+
+## Enabling a feature flag
+
+Check how to [roll out changes using feature flags](rolling_out_changes_using_feature_flags.md).
+