doc: announce API change for extended stats

doc: announce API change for extended stats

This patch adds a notice that the API for the xstats
functionality will be modified in the 16.07 release, with
no backwards compatibility planned as it would require
code duplication in each PMD that supports xstats.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Remy Horton <remy.horton@intel.com>
Acked-by: David Harton <dharton@cisco.com>
Acked-by: Maryam Tahhan <maryam.tahhan@intel.com>