LICENSES: add 'license-rules.txt'

LICENSES: add 'license-rules.txt'

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

This file is mainly copied from the Linux kernel file in
	Documentation/process/license-rules.rst
originally written by Thomas Gleixner <tglx@linutronix.de>, with
specific adaptations for OpenOCD.

Change-Id: I7a98fc756df90dc86dbc6e0c47c009a610a0318d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5973
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>