Rename master to main

[ericpre]

See https://github.com/github/renaming for details.

The master branch simply needs to be renamed but I open an issue to avoid surprises!

[jlaehne]

If no one objects, I would wait a few days and then do the rename.

[jlaehne]

Done.

Everyone locally still using the master branch now needs to rename their local clones:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[jlaehne]

And rename the branch name in your fork of course.