Change the default branch from `master` to `main`

This MR changes documentation references to the master branch that's about to get deleted. It also adds one-line notice at the top of the README to help contributors change their default branch.

No scripts are currently referencing master.

After this MR gets approved, we are going to close (not merge) the MR, set main as default branch and delete the master Then we'll propagate these changes to the security repo

Closes #214 (closed)