docs: add reference to trustedfirmware.org

This patch modifies the readme to add references to
the TF-RMM webpage in trustedfirmware.org.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I469fa0c700a3720c669e3f249dd29bf8a8119212
1 file changed