Drop requirement for CLA in contribution.md

It is no longer necessary for contributors to send a CLA to ARM
before making contributions. Contributors must instead add a
"Signed-off-by:" line to each commit, which certifies that the
contribution is made under the Developer Certificate of Origin
(DCO).

Update contributing.md to reflect this new policy and add a copy of
the DCO to the repository.

Change-Id: I7ca98bffc3bf57e8bdd51d763c24f13e415a328b
Signed-off-by: Dan Handley <dan.handley@arm.com>