Rename `master` branch to `main`

[jonkoops]

To some the name master can be considered offensive, the common practice has been to re-name branches from master to main, see the statement of the Software Freedom Conservancy.

[jonkoops]

@mafintosh since I don't have permissions to do this on the repo, can I ask you to rename the branch? This can be done quite easily from the GitHub UI by going to the branch overview, clicking the overflow menu on the master branch and selecting "rename branch".