Include LICENSE for the original R package

[dpsanders]

I think (part of?) the license for the original R package needs to be included somehow in the LICENSE file, or at least mentioned. I'm not familiar with the rules for GPL.

[dpsanders]

Part of the review for the JuliaCon proceedings: https://github.com/JuliaCon/proceedings-review/issues/96

[AnderGray]

Good spot. I've been in touch with the pba.r developers, and they said that if this was an issue, the GPL is probably not the best license for pba.r, let alone this package.

About 50% of the code is a port of R. And I've made explicit note of this in the header of each src file where this is relevant, including a link to the original R code. Since the R code is open-source on GitHub, I don't believe the original source code for that package needs to be distributed here. I have also made an explicit note of this in the LICENSE.

Eventually I'd like to completely recode those files to make it more Julian, and perhaps change the license to MIT. But hopefully for now you agree that this is an appropriate fix.