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

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Cc: larsi@gnus.org, gregory@heytings.org, stefan@marxist.se,
>       50880@debbugs.gnu.org
> Date: Sat, 02 Oct 2021 19:17:35 -0400
> 
> However, it may be necessary to give it a key binding
> so it can appear in the Help menu.  And having it appear there
> is useful.

We can have it in the Help menu without a key binding, there's no
requirement that it does have a binding for that.