The documentations only covered the percentage based feature gates.
These don't work really well for a number of reasons, for example; a new
visual element in GitLab would be weird to see only 25% of the time.
Allowing feature gates based on projects and groups has been supported,
now it's documented.
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.
Zeger-Jan van de Weg