Add documentation to README on working with git externals

In the top-level README: We should add a section equivalent to 2.3 "Modifying a component" but for a component in git, since it's important to point out the subtleties of remotes.

I am waiting to do this until NCAR/manage_externals#4 is resolved, because I'm not able to do this using my preferred workflow with that issue in place.