doc: add description of review tags

doc: add description of review tags

This commit details what is meant by the various email
tags that the DPDK community use regularly. The descriptions
state what each tag means, drawing from the kernel's understanding[1],
and the discussion on the DPDK mailing list[2].

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
[1]
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#when-to-use-acked-by-and-cc

[2] http://dpdk.org/ml/archives/dev/2017-January/thread.html#56300