This adds a development guide explaining that we are going to use
feature flags more often, why, what the benefits are, and so on.
See https://gitlab.com/gitlab-org/gitlab-ce/issues/49619 for more
information.
Previous code would not work with `disabled?` because that method would
send two parameters (second always `nil`) which we are not mocking.
Instead of mock yet another state, I decide to fix it where it belongs.
