Document the feature groups

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

doc/api/README.md

@@ -17,6 +17,7 @@ following locations:
 - [Deploy Keys](deploy_keys.md)
 - [Environments](environments.md)
 - [Events](events.md)
+- [Feature flags](features.md)
 - [Gitignores templates](templates/gitignores.md)
 - [GitLab CI Config templates](templates/gitlab_ci_ymls.md)
 - [Groups](groups.md)

doc/api/features.md

-# Features API
+# Features flags API
 
 All methods require administrator authorization.
 

doc/development/feature_flags.md

@@ -3,5 +3,19 @@
 Starting from GitLab 9.3 we support feature flags via
 [Flipper](https://github.com/jnunemaker/flipper/). You should use the `Feature`
 class (defined in `lib/feature.rb`) in your code to get, set and list feature
-flags. During runtime you can set the values for the gates via the
-[admin API](../api/features.md).
+flags.
+
+During runtime you can set the values for the gates via the
+[features API](../api/features.md) (accessible to admins only).
+
+## Feature groups
+
+Starting from GitLab 9.4 we support feature groups via
+[Flipper groups](https://github.com/jnunemaker/flipper/blob/v0.10.2/docs/Gates.md#2-group).
+
+Feature groups must be defined statically in `lib/feature.rb` (in the `.register_feature_groups` method), but their implementation can obviously be
+dynamic (querying the DB etc.). You can see how the `performance_team` feature
+group for a concrete example.
+
+Once defined in `lib/feature.rb`, you will be able to activate a
+feature for a given feature group via the [`feature_group` param of the features API](../api/features.md#set-or-create-a-feature)