Default branch renamed: `master` is now `main`

I renamed the default branch `master` to `main`

## Why?

To follow the (defacto) industry standard.
Read more here: https://github.com/github/renaming

## What to do if you have a local copy?

Execute

```
git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a
```