Fix/license for cran

I'm not certain that this is the right incantation but taking a bit of inspiration from e.g. 

<http://cran.rstudio.com/web/packages/optparse/COPYRIGHTS>

I can offer this small patch to get things rolling again.
This is not the right place to discuss, but I guess it would make sense to create a CLA (http://en.wikipedia.org/wiki/Contributor_license_agreement) for rappdirs (and other projects). Just to be safe in the future. Anyway, just bringing this up, it's up to the owner to decide, obviously.