Move default branch from "master" -> "main"

@jrbourbeau and I are in the process of moving the default branch for this repo from master to main.

- [ ] Changed in GitHub
- [ ] Merged PR to change branch name in code (xref #43)

### What you'll see
Once the name on github is changed (the first box above is Xed, or this issue closed), when you try to `git pull` you'll get

```bash
Your configuration specifies to merge with the ref 'refs/heads/master'
from the remote, but no such ref was fetched.
```

### What you need to do

First: **head to your fork and rename the default branch there**
Then:
```bash
git branch -m master main
git fetch origin
git branch -u origin/main main
```