first pass on novice r reference sheet

addresses #663

First pass on the reference sheet.

almost just typed in any function/code I saw

Lots of work to do, but it's a start.
Need to cover indexing examples for atomic vectors, matrices, and lists here (`[` and `[[`, also `$`) if we are going to include this. I think this would be a useful topic to include as a handy aide memoir for the different types of subsetting operations