Update README to recommend librdkafka 0.8.6+
Update README to recommend librdkafka 0.8.6+

Bottled Water relies on null message payloads for log compaction correctness; however librdkafka only added support for null messages in release 0.8.6.  This updates the readme to recommend 0.8.6+.  Since both the official Ubuntu librdkafka packages, and the PPA bottledwater package, predate 0.8.6, this also recommends against using those packages until they are updated.
confluentinc/bottledwater-pg,confluentinc/bottledwater-pg,confluentinc/bottledwater-pg,confluentinc/bottledwater-pg