Update README to suggest running a git fetch and branch cleanup

[C-Howard]

This pr is an attempt to document the post merge work / branch cleanup. I made my best attempt to write valid documentation, I am not personally setup to run emacs on my computer. I attempted to use documentation and examples of formatting in the README now to try to get it as right as possible. This is a bit of a rough draft as I made have missed some important information.

Closes #55