Add gitlab workflow to untranslatable pages

Summary:
This patch adds the description of how one should
use labels to control the flow of merge requests.
This patch also contains information how should
one prepare commits before merging with master.

This should've gone into master, but the patch won't
apply so we're putting it in draft for now. Draft will
be merged soonish anyway.

Differential Revision: https://phabricator.kde.org/D20759