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 <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

COPYING.new

···
+The Linux Kernel is provided under:
+
+	SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note
+
+Being under the terms of the GNU General Public License version 2 only,
+according with:
+
+	LICENSES/preferred/GPL-2.0
+
+With an explicit syscall exception, as stated at:
+
+	LICENSES/exceptions/Linux-syscall-note
+
+In addition, other licenses may also apply. Please see:
+
+	Documentation/process/license-rules.rst
+
+for more details.

Documentation/process/license-rules.rst

···
 ============================
 
 The Linux Kernel is provided under the terms of the GNU General Public
-License version 2 only (GPL-2.0), as published by the Free Software
-Foundation, and provided in the COPYING file.  This documentation file is
-not meant to replace the COPYING file, but provides a description of how
-each source file should be annotated to make the licensing it is governed
-under clear and unambiguous.
+License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0,
+with an explicit syscall exception described in
+LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file.
 
-The license in the COPYING file applies to the kernel source as a whole,
-though individual source files can have a different license which is
-required to be compatible with the GPL-2.0::
+This documentation file provides a description of how each source file
+should be annotated to make its license clear and unambiguous.
+It doesn't replace the Kernel's license.
+
+The license described in the COPYING file applies to the kernel source
+as a whole, though individual source files can have a different license
+which is required to be compatible with the GPL-2.0::
 
     GPL-1.0+  :  GNU General Public License v1.0 or later
     GPL-2.0+  :  GNU General Public License v2.0 or later