changed README and change CI to reflect new main branch

[edwardhartnett]

DESCRIPTION OF CHANGES:

Updated README, also changing CI system, to handle new branch main.

TESTS CONDUCTED:

N/A

Machines/Platforms:

N/A

Test cases:

N/A

ISSUE:

Part of #469

[edwardhartnett]

@MatthewPyle-NOAA after this is merged, go back to settings, and under branch protection you will be able to type in "docs" and "flake8" to make these required CI runs. That way, GitHub will enforce that these work.

[MatthewPyle-NOAA]

@edwardhartnett I made a small addition to the README file - let me know if it is/isn't okay with you.

[MatthewPyle-NOAA]

Approving the first PR to "main"...

[MatthewPyle-NOAA]

@MatthewPyle-NOAA after this is merged, go back to settings, and under branch protection you will be able to type in "docs" and "flake8" to make these required CI runs. That way, GitHub will enforce that these work.

@edwardhartnett Is this adding new branch protection rules? Not intuitive to me where I need to add "docs" and "flake8"