board: freescale: t208xrdb: Add link to User Guide

The User Guide contains handy things like block diagrams and DIP switch
settings and it's even available on the public web. Add a link to it in
the README.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>