Created for example when using https://github.com/jennybc/googlesheets

Details: `RData` is a valid file format for bundling data with an R
package based on the "Writing R Extensions" manual:
http://cran.r-project.org/doc/manuals/r-release/R-exts.html#Data-in-packages

.Rproj is fine - it's .Rproj.user that needs to go.

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.