doc: add details on requirements for patch ack and merge

doc: add details on requirements for patch ack and merge

Add to the contributors guide the requirements and guidelines for
getting patches acked and merged. It details at what point the review
comments and the ack's need to be received in order to have a given
patch merged into a release.

These guidelines are as agreed by the DPDK technical board at the
meeting held on 2017-02-15.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: John McNamara <john.mcnamara@intel.com>