2.16: Mention problem with plugin and online reindex

The fix for online reindexing with plugins has not been submitted
yet, but we're going to release 2.16.1 soon. Add a section mentioning
the known issue and include workarounds for the problem with the
replication plugin.

Change-Id: I3acc650739f8bd16c674180e3072db4a05d523da
1 file changed