bug#50880: Add "Contributing to Emacs" help command (see C-h C-h)

[Gregory Heytings]

> If 'C-h C-x' is too <important/expensive/etc.> to use for speculating in 
> recruiting contributors, I suggest we close this as wontfix instead.

There's still one solution that should satisfy everyone, because it does 
not "consume" any of the still free key bindings.  It would give the 
following section in help-for-help (aka C-h C-h):

Miscellaneous

C-a About Emacs
C-f Emacs FAQ
C-n News of recent changes
C-m Order printed manuals

C-p Known problems
C-d Debugging Emacs
C-c Contribute improvements to Emacs
C-t Emacs TODO

g   About the GNU project
G   Emacs copying permission (GNU General Public License)
C-o Emacs ordering and distribution information
C-w Information on absence of warranty