missing permissions in reference manual copyright page (PR#3528)

Brian:

Several weeks ago you posted about the missing permissions in the R
reference manual, and asked whether the manual is freely
distributable.  You didn't get a reply, I think because everyone is
travelling so much this summer.

The answer is yes, the manual is covered under the same GPL as the
rest of R.  I've added the paragraphs to the reference manual source
in the patch version of 1.7.1; they should migrate to 1.8.0 as well.

Thanks for sending them.

Duncan Murdoch