infra/rdn1edgex2: add user guide for rd-n1-edge dual-chip platform

Add the initial user guide for the RD-N1-Edge Dual-Chip platform
software stack. This version 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: I3668544148e33f93c3cad420ac5547f2493df974