Document master branch in https://tails.boum.org/contribute/git/
Originally created by @emmapeel on #9528 (Redmine)
It would be great to replace the FIXME tag with information about the branch
https://tails.boum.org/contribute/git/#index2h3
FIXME: document what should go in there, when other branches are merged into it, etc.
Feature Branch: emmapeel:feature/9528-document_master_branch