README: Updating default branch of local clones to `main` from 7 Dec, 2020

[Larene]

If you have a fork or local clone from before 7 Dec 2020, you may need to run the following commands to update your forked/cloned repo.

//you might need to do: git fetch origin
git checkout master
git branch -m master main
git fetch
git branch --unset-upstream
//if fork: git push -u origin main
git branch -u origin/main
git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main