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

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Wed, 29 Sep 2021 19:55:34 +0300
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 50880@debbugs.gnu.org,
>  Gregory Heytings <gregory@heytings.org>
> 
> Here is another problem:
> 
>   TAB  Show all installed manuals
> 
> should be
> 
>   C-i  Show all installed manuals
> 
> because the mnemonic key is 'C-i'.

If C-i was selected because the 'i' part, then it's too bad, because
the correct name is TAB, for the same reason we use RET abd BS and DEL
and ESC instead of their Ctrl equivalents.