quickstart: remind users of the compilation duration

Add note to inform users compilation of dataplane-stack might take some
time.

Signed-off-by: Jieqiang Wang <jieqiang.wang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I127a736d953469b9b47fec6f34b321238f1f61f4