firmware: Update information in linux.git about adding firmware

Kyle McMartin joined the linux-firmware maintainers, and we now
have an alias <linux-firmware@kernel.org> which reaches all of us.
Include that instead of the individual addresses.

Add some further recommendations that were already included in the
README in linux-firmware.git added by Xose Vazquez Perez
<xose.vazquez@gmail.com>.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>