[Bug-gsl] Add information for subscribing to the gsl-discuss list

[Daniel Webb]

The "Further information" section in the manual:

http://www.gnu.org/software/gsl/manual/gsl-ref_toc.html#TOC7

"The developers of the library can be reached via the development mailing
list address@hidden"

No instructions are given on how to subscribe to this list.  Since the
subscription method is different than the mailing list described just a
few lines previous, it should be spelled out.

Daniel