doc: add guideline for updating release notes
doc: add guideline for updating release notes

From version 2.2 of DPDK onwards patchsets should include
updates to the Release Notes for additions, fixes and
changes.

Add guideline on what to update in the Release Notes to the
Documentation Contribution guidelines.

Signed-off-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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