infra/rdv1: add user guide for rd-v1 platform

Add the initial user guide for the RD-V1 platform software stack. The
user guide points to documents that provide information about obtaining
the platform software stack and using the various features it supports.
A table to list the release tags is included as well.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: Iec4dcba16aedf98916078473dfc21288917cce3c