app_dirs -> app_dir in README.md

* not sure if the function was ever called app_dirs but it is not presently
Add Rstudio project
re-roxygenise with roxygen2 v4.0.1
Add LICENSE file

Needed by R CMD check (R-3.1.0).
Add package docs

* provides basic info about the package and ensures that ?rappdirs works.
import rather than depend on methods

* imports rather than depends should be sufficient
* explicit import in NAMESPACE needed to make R 3.1.0 R CMD CHECK happy
Add Gabor Csardi and myself as contributors