Documentation: Add license-rules.rst to describe how to properly identify file licenses

Add a file to the Documentation directory to describe how file licenses
should be described in all kernel files, using the SPDX identifier, as well
as where all licenses should be in the kernel source tree for people to
refer to (LICENSES/).

Thanks to Kate and Greg for review and editing and Jonas for the
suggestions concerning the meta tags in the licenses files.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jonas Oberg <jonas@fsfe.org>
Reviewed-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>