Rename master branch to main

[justinsalamon]

"master" isn't an inclusive term (cf. https://www.zdnet.com/article/github-to-replace-master-with-main-starting-next-month/#:~:text=Starting%20next%20month%2C%20all%20new,them%20with%20more%20inclusive%20terms.)

By the end of the year making this move should be easy, hopefully: https://github.com/github/renaming