WriteFreely branch structure

Now that we're growing past one core developer, we've moved #WriteFreely development off of master. As I mentioned on Mastodon:

Things are validated. Now all core development is done on feature branches off of develop, with code review before merging. (So, more validated than the master branch used to be.)

Of course, there may be some larger features that get added over several pull requests (thus might need fixes / changes), but in general we're trying to avoid that.