Update HISTORY.md with the recent memtable trimming fixes

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6194

Differential Revision: D19125292

Pulled By: ltamasi

fbshipit-source-id: d41aca2755ec4bec07feedd6b561e8d18606a931

HISTORY.md

@@ -7,6 +7,7 @@
 ### Bug Fixes
 * Fix a bug that can cause unnecessary bg thread to be scheduled(#6104).
 * Fix a bug in which a snapshot read could be affected by a DeleteRange after the snapshot (#6062).
+* Fixed two performance issues related to memtable history trimming. First, a new SuperVersion is now created only if some memtables were actually trimmed. Second, trimming is only scheduled if there is at least one flushed memtable that is kept in memory for the purposes of transaction conflict checking.
 
 ## 6.6.0 (11/25/2019)
 ### Bug Fixes