doc: add deprecation notice for external memory

doc: add deprecation notice for external memory

Due to the upcoming external memory support [1], some API and ABI
changes will be required. In addition, although the changes called
out in the deprecation notice are not yet present in form of code
in the published RFC itself, they are based on consensus on the
mailing list [2] on how to best implement this feature.

[1] http://patches.dpdk.org/project/dpdk/list/?series=453&state=*
[2] https://mails.dpdk.org/archives/dev/2018-July/108002.html

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Keith Wiles <keith.wiles@intel.com>
Acked-by: Zhihong Wang <zhihong.wang@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Shreyansh Jain <shreyansh.jain@nxp.com>