speakup: Add documentation on changing the speakup messages language

This documents how to use speakup_setlocale to set the speakup messages
language.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20210126222147.3848175-5-samuel.thibault@ens-lyon.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>