Add new lesson novice/r/rstudio/git

See #492
Here is a screenshot I usually link to for the Windows folks: <http://www.molecularecologist.com/wp-content/uploads/2013/11/Screenshot-2013-11-12-09.53.56-Copy1.png>.  It is from a blog post [Using GitHub with R and RStudio](http://www.molecularecologist.com/2013/11/using-github-with-r-and-rstudio/)