Opening SCP-firmware to external contributions

With this patch, the SCP-firmware will accept contributions to the project
from non-Arm contributors. Contributions must be made under Developer
Certificate of Origin (DCO).

This patch updates the contributing guidelines to describe how contributions
can be made, and additionally adds a copy of the DCO terms.

Change-Id: I1cb23b1785c4fb074ae0c344556319a2d7a0254e
Signed-off-by: David Cunado <david.cunado@arm.com>