Update HISTORY.md for GetThreadList() update.

Summary: Update HISTORY.md for GetThreadList() update.

Test Plan: no code change

Reviewers: sdong, rven, anthony, krishnanm86, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D38685

HISTORY.md

@@ -8,6 +8,7 @@
 ### New Features
 * Added an experimental API for handling flashcache devices (blacklists background threads from caching their reads) -- NewFlashcacheAwareEnv
 * If universal compaction is used and options.num_levels > 1, compact files are tried to be stored in none-L0 with smaller files based on options.target_file_size_base. The limitation of DB size when using universal compaction is greatly mitigated by using more levels. You can set num_levels = 1 to make universal compaction behave as before. If you set num_levels > 1 and want to roll back to a previous version, you need to compact all files to a big file in level 0 (by setting target_file_size_base to be large and CompactRange(<cf_handle>, nullptr, nullptr, true, 0) and reopen the DB with the same version to rewrite the manifest, and then you can open it using previous releases.
+* More information about rocksdb background threads are available in Env::GetThreadList(), including the number of bytes read / written by a compaction job, mem-table size and current number of bytes written by a flush job and many more.  Check include/rocksdb/thread_status.h for more detail.
 
 ### Public API changes
 * TablePropertiesCollector::AddUserKey() is added to replace TablePropertiesCollector::Add(). AddUserKey() exposes key type, sequence number and file size up to now to users.