update history for recent commits

Summary:
I browsed through the history since 5.9 was released and found some changes worth mentioning in HISTORY.md.
Closes https://github.com/facebook/rocksdb/pull/3237

Differential Revision: D6506472

Pulled By: ajkr

fbshipit-source-id: 627ce9f94ca33df9f0f231a9c5ced3624b05506c

HISTORY.md

@@ -2,9 +2,15 @@
 ## Unreleased
 ### Public API Change
 ### New Features
+* Provide lifetime hints when writing files on Linux. This reduces hardware write-amp on storage devices supporting multiple streams.
+* Add a DB stat, `NUMBER_ITER_SKIP`, which returns how many internal keys were skipped during iterations (e.g., due to being tombstones or duplicate versions of a key).
+* Add PerfContext counters, `key_lock_wait_count` and `key_lock_wait_time`, which measure the number of times transactions wait on key locks and total amount of time waiting.
+
 ### Bug Fixes
 * Fix IOError on WAL write doesn't propagate to write group follower
 * Make iterator invalid on merge error.
+* Fix performance issue in `IngestExternalFile()` affecting databases with large number of SST files.
+* Fix possible corruption to LSM structure when `DeleteFilesInRange()` deletes a subset of files spanned by a `DeleteRange()` marker.
 
 ## 5.9.0 (11/1/2017)
 ### Public API Change