master branch renamed to main

[bbfrederick]

Is your feature request related to a problem? Please describe.
The evolving standard on GitHub is to name the default branch "main", rather than "master". rapidtide was started using the old standard.

Describe the solution you'd like
Rename master branch to main

This is more a notification than anything else. I went ahead and did it, following the instructions at https://github.com/github/renaming.

New downloads will have two branches, main (default), and dev. To update existing local repositories, go to the directory and type:

git branch -m master main
git fetch origin
git branch -u origin/main main