Please document `contrib/download_prerequisites'

[Gerald Pfeifer]

On Thu, 30 May 2013, Michael Witten wrote:
>> http://gcc.gnu.org/wiki/InstallingGCC
> I'm not looking for anything.
> 
> That wiki information should be incorporated into what that wiki page
> calls `the official installation docs', and the rest of it should
> probably be thrown out as superfluous.

I agree.  Are you willing to help with this?  I commit to
support you (review/approve/commit) patches with "some or
next day" turnaround whenever I can.

Gerald