add instructions for changing editor for commit messages

[jesse-ross]

I proctored this course for a user who was doing everything through git bash rather than RStudio. They do not have any familiarity with vim. Everything was going fine because they were doing git commit -m, but when they hit a merge conflict they were dropped into a vim session with no idea how to edit the commit message or quit vim.

I think it would be worthwhile to add a section on editor config, to keep people from getting trapped here. For him, git config core.editor notepad did the trick. Not sure what the most widespread editor is for Mac users, but happy to research and add.