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.
See http://cran.rstudio.com/web/packages/optparse/COPYRIGHTS (also by @trevorld) which obviously made it through CRAN once upon a time ...