Add description to the 2PC checkpooint bug in HISTORY.md

Summary: Closes https://github.com/facebook/rocksdb/pull/1729

Differential Revision: D4371674

Pulled By: siying

fbshipit-source-id: 907e373

HISTORY.md

@@ -5,6 +5,9 @@
 * Added EventListener::OnExternalFileIngested which will be called when IngestExternalFile() add a file successfully.
 * BackupEngine::Open and BackupEngineReadOnly::Open now always return error statuses matching those of the backup Env.
 
+### Bug Fixes
+* Fix the bug that if 2PC is enabled, checkpoints may loss some recent transactions.
+
 ## 5.0.0 (11/17/2016)
 ### Public API Change
 * Options::max_bytes_for_level_multiplier is now a double along with all getters and setters.