Rename default branch from `master` to `main`

[mrclmr]

This is not so important but it would help to be more conform with the convention and would lower a tiny bit the bar for contributors to this repository.

• All new GitHub repositories use main as the default branch
• git init gives a hint to change the default branch to main

  hint: Using 'master' as the name for the initial branch. This default branch name
  hint: is subject to change. To configure the initial branch name to use in all
  hint: of your new repositories, which will suppress this warning, call:
  hint:
  hint: 	git config --global init.defaultBranch <name>
  hint:
  hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
  hint: 'development'. The just-created branch can be renamed via this command:
  hint:
  hint: 	git branch -m <name>
  

• I am used to type mai and hit TAB to let my shell complete to main
• main could give the contributors the impression that the repository is more up-to-date

Here you can go to the branches of the repository (I didn't find it myself):

After you changed the default branch name to main GitHub will help you with suitable command hints to reflect the change locally as well.