doc: mention 17.11 LTS in contributing guide
doc: mention 17.11 LTS in contributing guide

Signed-off-by: Luca Boccassi <020748dc039465edc3ee6f30c530b21164cdce27@debian.org>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>
Acked-by: Thomas Monjalon <5f50a84c1fa3bcff146405017f36aec1a10a9e38@monjalon.net>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk