Add notice commit indicating the default branch is now main

This MR adds an empty commit patch with the notice message below as the commit message. This will become the final commit on the master branch. If users accidentally continue to try to use the old branch, they can more easily see that they need to change their local git config for the branch change. (Ref: #50 (closed))

To update your local repository to use the new default branch, these
commands may help:

$ git fetch origin
$ git checkout master
$ git branch -m main
$ git branch --set-upstream-to=origin/main