Re-add Feature.register_feature_groups which is already documented

This is a follow-up for !12362 where this was documented but the code
was removed in the last iteration. Since this can still be useful and
this is already supported by the API, I think re-adding the code was
the best course of action.
Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/flipper.rb

@@ -3,4 +3,6 @@ require 'flipper/middleware/memoizer'
 unless Rails.env.test?
   Rails.application.config.middleware.use Flipper::Middleware::Memoizer,
     lambda { Feature.flipper }
+
+  Feature.register_feature_groups
 end

lib/feature.rb

@@ -57,5 +57,11 @@ class Feature
         Flipper.new(adapter)
       end
     end
+
+    # This method is called from config/initializers/flipper.rb and can be used
+    # to register Flipper groups.
+    # See https://docs.gitlab.com/ee/development/feature_flags.html#feature-groups
+    def register_feature_groups
+    end
   end
 end