These patterns handle cache diretories and temporary files created
by the widely used [knitr](http://yihui.name/knitr/) and [R markdown](http://rmarkdown.rstudio.com/)
packages.

N.B. this ignores only the exact file `.RData` generated by default from `save.image()` and `quit()`.  It is _not_ what is referred to in https://github.com/github/gitignore/commit/1f8466164330277fc8c9126ded3cc207ff3baf91 and http://cran.r-project.org/doc/manuals/r-release/R-exts.html#Data-in-packages, which refers only to files with an `*.RData` _extension_.

Both rules are restricted to the top level only, so `*.tar.gz` files in data directories for instance will _not_ be ignored.

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.