Add branch renaming notice

BRANCHRENAME.md

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+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)