We renamed `master` branch to `main` 💥 (Migration guide)

Hey all,

today we have renamed our default branch `master` to `main`.
Why? This is mainly described in https://github.com/github/renaming

## How to migrate

If you have a local version of go-jira, here is how you can migrate:

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

## Additional information

This issue is just for informational purposes.
I will close this in a couple of weeks.