Add git config to Get Set instructions

[osteele]

Get Set instructions should include these:
git config --global push.default simple
git config --global user.name "Frankly Olin"
git config --global user.email frank@students.olin.edu
git config --global core.editor atom
git config --global credential.helper 'cache --timeout=3600'

More reading: https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration

Consider these:
git config --global core.excludesfile ~/.gitignore_global
git config --global core.whitespace
trailing-space,-space-before-tab,indent-with-non-tab,tab-in-indent,cr-at-eol