rename branch master to main

[MathewBiddle]

The Software Freedom Conservancy released a statement (https://sfconservancy.org/news/2020/jun/23/gitbranchname/) in 2020 detailing why they are encouraging projects to shift from "master" to "main" and lay out some good reasons for it; GitHub has gone the same direction (https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/), as have several other git hosters.

This issue is a recommendation to adjust the default branch name from master to main.

[thomas-robinson]

I am in the NOAA Mid Career Leadership Development Program (MCLDP) and I am going to push for this in a NOAA-wide sense. I think this is a great idea and we can try to coordinate changing the branch names across NOAA.