docs: adding instructions to assign IP addresses

Signed-off-by: Jay Wang <Jay.Wang2@arm.com>
Reviewed-by: Luca Vizzarro <Luca.Vizzarro@arm.com>
Reviewed-by: Paul Szczepanek <Paul.Szczepanek@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>
Change-Id: Ifb13970fa3bbd4290b81346aa897cbf49a6ad919