Rename the "master" branch.

[b-abel]

In the light of current debates, many projects are renaming the default branch from "master" to something more appropriate. Github is apparently also working on changing the default name.
The name "main" seems to gain ground as an alternative.

While the change as such is quickly done, the effort increases if there are external dependencies like HTML links or open PRs.
This repository is new, so it is IMO the perfect time for this change.

[b-abel]

The changes in Github, Git itself, and the ecosystem can be tracked here.