doc: announce mempool API changes

doc: announce mempool API changes

An API/ABI changes are planned for 18.05 [1]:

 * Allow to customize how mempool objects are stored in memory.
 * Deprecate mempool XMEM API.
 * Add mempool driver ops to get information from mempool driver and
   dequeue contiguous blocks of objects if driver supports it.

[1] http://dpdk.org/ml/archives/dev/2018-January/088698.html

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>