infra/rdn1edge: add user guide for rd-n1-edge platform

Add the initial user guide for the RD-N1-Edge platform software stack. The
version of of the user guide points to documents that provide information
about how to obtain the platform software stack and use the various features
it provides.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Change-Id: I79ac797c65067c8f5001ab13e6136d11d4f7baa8