PluralKit/BRANCHRENAME.md

On 2020-06-22, we renamed the master branch to main.

This should mean very little, but if you have existing clones, you'll need to update the branch references, like so:

1. Fetch the latest branches from the remote:
   $ git fetch --all
2. Update the upstream remote's HEAD
   $ git remote set-head origin -a
3. Switch your local branch to track the new remote
   $ git branch --set-upstream-to origin/main
4. Rename your branch locally
   $ git branch -m master main

(steps from https://dev.to/rhymu8354/git-renaming-the-master-branch-137b)

I'm going to re-branch master from main, leaving it at this notice's commit, and then delete in a week's time so people have a chance to migrate. Hopefully this doesn't cause too much breakage. (if it does, do yell at me in the issues)