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

[Stefan Kangas]

Juri Linkov <juri@linkov.net> writes:

> Here is another problem:
>
>   TAB  Show all installed manuals
>
> should be
>
>   C-i  Show all installed manuals
>
> because the mnemonic key is 'C-i'.

Yup.

This is a limitation in `substitute-command-keys' that I think we
should fix.  Feel free to open a bug report about it.