Merge branch 'gollum-initializer-warning' into 'master'

Add notice to Gollum initializer

See merge request gitlab-org/gitlab-ce!15713

config/initializers/gollum.rb

+# WARNING changes in this file must be manually propagated to gitaly-ruby.
+#
+# https://gitlab.com/gitlab-org/gitaly/blob/master/ruby/lib/gitlab/gollum.rb
+
 module Gollum
   GIT_ADAPTER = "rugged".freeze
 end