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>

Documentation/index.rst

@@ -13,6 +13,18 @@ documents into a coherent whole.  Please note that improvements to the
 documentation are welcome; join the linux-doc list at vger.kernel.org if
 you want to help out.
 
+Licensing documentation
+-----------------------
+
+The following describes the license of the Linux kernel source code
+(GPLv2), how to properly mark the license of individual files in the source
+tree, as well as links to the full license text.
+
+.. toctree::
+   :maxdepth: 2
+
+   process/license-rules.rst
+
 User-oriented documentation
 ---------------------------