Add license to the release documentation and requirements doc

Add license to the release documentation and requirements doc

Documentation in OPNFV has to be licensed under the
Creative Commons Attribution 4.0 International License. This patch adds
the license header to the .rst files for the release documentation. A
LICENSE file is also added to the folder that contains images.

Change-Id: I207a07830246fcc103c081d385dc9b12f47ddfb3
Signed-off-by: Ildiko Vancsa <ildiko.vancsa@ericsson.com>
(cherry picked from commit d872ca8e95d82cb90069ee25d625bbe8e985dcb8)