Add *.Rproj for RStudio users

RStudio - the only real R IDE, and an incredibly commonly used environment for R, particularly since it has git support - uses .Rproj files in projects, that follow the pattern projectname.Rproj. Adding *.Rproj to capture these and exclude them.
Merge pull request #1183 from Ironholds/patch-1

Add *.Rproj for RStudio users