Release note for partitioned index/filters

Summary: Closes https://github.com/facebook/rocksdb/pull/2637

Differential Revision: D5489751

Pulled By: maysamyabandeh

fbshipit-source-id: 0298f8960d4f86ce67959616615beee4d802c2e4

HISTORY.md

@@ -12,6 +12,8 @@
 * RateLimiter support for throttling background reads, or throttling the sum of background reads and writes. This can give more predictable I/O usage when compaction reads more data than it writes, e.g., due to lots of deletions.
 * [Experimental] FIFO compaction with TTL support. It can be enabled by setting CompactionOptionsFIFO.ttl > 0.
 * Introduce `EventListener::OnBackgroundError()` callback. Users can implement it to be notified of errors causing the DB to enter read-only mode, and optionally override them.
+* Partitioned Index/Filters exiting the experimental mode. To enable partitioned indexes set index_type to kTwoLevelIndexSearch and to further enable partitioned filters set partition_filters to true. To configure the partition size set metadata_block_size.
+
 
 ### Bug Fixes
 * Fix discarding empty compaction output files when `DeleteRange()` is used together with subcompactions.