infra/rdn2cfg2: add user guide for RD-N2-Cfg2 platform

Add the initial user guide for the RD-N2-Cfg2 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: Zakaria Zahi <zakaria.zahi@arm.com>
Change-Id: I222b82370437ba588a675b1e509ad347d426a432