Update HISTORY.md

Summary: Updating HISTORY.md to reflect changes in PlainTable index

Test Plan: no need

Reviewers: sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20229

HISTORY.md

@@ -3,6 +3,7 @@
 ### Unrelease
 ### New Features
 * Support Multiple DB paths in universal style compactions
+* Add feature of storing plain table index and bloom filter in SST file.
 
 ### Public API changes
 * DBOptions.db_paths now is a vector of a DBPath structure which indicates both of path and target size