Add license info guidance to docs

Extends the Contributing page to provide more guidance on adding
and updating license and copyright information. Added link to
coding style page where some example license and copyright comment
blocks have been added.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: I139f1300420d32c04686ddf1dbf450fa5a7484e8
2 files changed