[docs] added a note about NVM-based cache into known issues
[docs] added a note about NVM-based cache into known issues

The NVM-based cache doesn't work reliably on RH6/CentOS6 once
memkind was introduced.  See KUDU-2978 for details.

Change-Id: Ieca7f77f1401bcef2af1242fc0090c62c0bb45f6
Reviewed-on: http://gerrit.cloudera.org:8080/14510
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Kudu Jenkins

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu