infra/sgi575: add user guide for sgi-575 platform

Add the initial user guide for the SGI-575 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: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I4730ee89ece766577708126cec3cf0bc9e878bc3