Docs: Clarify copyright requirements

Clarify copyright requirements in contributing.md.

Also clarify maintainership structure by adding a new maintainers.md. This
imports individual maintainer details from the GitHub wiki.

Fixes ARM-software/tf-issues#488

Change-Id: I7135d3f77ea45533f667de7e1dcdf65697486a91
Signed-off-by: Dan Handley <dan.handley@arm.com>