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

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> `C-g' is more mnemonic, but I'm not enthusiastic about moving around key
> bindings that have been around this long.  (Even if it's not a command
> many people are actually using.)

I guess you will in any case only ever press this after looking at
`C-h C-h' first, so I don't think this matters really.

We could pick any key, like "b" for "contriButing".  Or, you know, "h"
for "Happens to be free".  ;-)