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>

Fix typo in the word "contributions".

Change-Id: I3051031a981bf0104b2b5a849a8e28090bcbd592
Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>

Please see doc/framework.md for details.

Co-authored-by: Filipe Rinaldi <filipe.rinaldi@arm.com>
Co-authored-by: Paul Beesley <paul.beesley@arm.com>
Co-authored-by: Chris Kay <chris.kay@arm.com>
Co-authored-by: Elieva Pignat <elieva.pignat@arm.com>
Co-authored-by: Pedro Custodio <pedro.krewinkelcustodio@arm.com>

Change-Id: I1744c60eca295c60b3fabcfafb522fbff5d7306c
Signed-off-by: Ronald Cron <ronald.cron@arm.com>