Update HISTORY.md (#4753)

Summary:
As titled. Update history to include a recent bug fix in
9be3e6b488.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4753

Differential Revision: D13350286

Pulled By: riversand963

fbshipit-source-id: b6324780dee4cb1757bc2209403a08531c150c08

HISTORY.md

@@ -1,5 +1,11 @@
 # Rocksdb Change Log
 ## Unreleased
+### New Features
+
+### Public API Change
+
+### Bug Fixes
+* Fix a deadlock caused by compaction and file ingestion waiting for each other in the event of write stalls.
 
 ## 5.18.0 (11/30/2018)
 ### New Features