[RFA/doco 3/4] gdb/README: Document the --with-libgmp-prefix configure option

[Eli Zaretskii]

> From: Joel Brobecker <brobecker@adacore.com>
> Date: Sun, 22 Nov 2020 15:56:29 +0400
> Cc: Joel Brobecker <brobecker@adacore.com>
> 
> gdb/ChangeLog:
> 
>         * README: Document the --with-libgmp-prefix configure option.

OK, thanks.