Commented .gitconfig.  Added cm alias

I'm opening the request a little early in the hopes you guys have a link or two to explain this stuff in more depth.  Googling didn't find my any clear use guides for students to look at.  I'm fairly sure of my comments, but please take time to make sure i'm not spreading misinformation.  

I also would like a clearer breakdown of push options.  I would like to have remote branch tracking while only pushing the current branch... if this is an option i'm not sure which one it is.  TY