doc: add guidelines for initial PMD submission

doc: add guidelines for initial PMD submission

This patch for DPDK Contributor's Guidelines indicates the repos
against which a new PMD should be prepared; for example, for new
network ethernet PMDs it should be dpdk-next-net, and for new crypto
PMDs it should be dpdk-next-crypto. For other new PMDs, the
contributor should refer to the MAINTAINERS file. Though this may seem
obvious, it is not mentioned in DPDK documentation.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Last contribution, last commit closing the release.
Thank you Rami, you will be missed.