COPYING: create a new file with points to the Kernel license files

With the addition of SPDX patchset, the contents of COPYING file
is now duplicated at two other files under LICENSE:
	LICENSES/preferred/GPL-2.0
	LICENSES/exceptions/Linux-syscall-note

It is easy to check that the contents of the licence written on
those files are identical with COPYING using:

	$ diff -upr COPYING LICENSES/preferred/GPL-2.0
	$ diff -upr COPYING LICENSES/exceptions/Linux-syscall-note|less

Also, a new file was added, with describes how SPDX should work at
the Kernel source files:
	Documentation/process/license-rules.rst

Instead fo having it copying the contents of two files, and not
even mentioning the third one, replace it by a file whose content
points to the other tree files, preserving the Kernel's license.

Adjust license-rules.rst accordingly.

Please notice that this file preserves the Kernel license as
is, without any changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>