Update HISTORY file for transactions

Summary: Move line about pessimistic transactions to the correct place in the history file and no longer refer to it as 'experimental'

Test Plan: n/a

Reviewers: igor, sdong, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46449

HISTORY.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 ### New Features
+* Added support for transactions.  See include/rocksdb/utilities/transaction.h for more info.
 * DB::GetProperty() now accept "rocksdb.aggregated-table-properties" and "rocksdb.aggregated-table-properties-at-levelN", in which case it returns aggregated table properties of the target column family, or the aggregated table properties of the specified level N if the "at-level" version is used.
 * Add compression option kZSTDNotFinalCompression for people to experiment ZSTD although its format is not finalized.
 * We removed the need for LATEST_BACKUP file in BackupEngine. We still keep writing it when we create new backups (because of backward compatibility), but we don't read it anymore.
@@ -11,6 +12,7 @@
 * Removed class Env::RandomRWFile and Env::NewRandomRWFile().
 * Renamed DBOptions.num_subcompactions to DBOptions.max_subcompactions to make the name better match the actual functionality of the option.
 * Added Equal() method to the Comparator interface that can optionally be overwritten in cases where equality comparisons can be done more efficiently than three-way comparisons.
+* Previous 'experimental' OptimisticTransaction class has been replaced by Transaction class.
 
 ## 3.13.0 (8/6/2015)
 ### New Features
@@ -29,7 +31,6 @@
 
 ## 3.12.0 (7/2/2015)
 ### New Features
-* Added experimental support for pessimistic transactions.  See include/rocksdb/utilities/transaction.h for more info.
 * Added experimental support for optimistic transactions.  See include/rocksdb/utilities/optimistic_transaction.h for more info.
 * Added a new way to report QPS from db_bench (check out --report_file and --report_interval_seconds)
 * Added a cache for individual rows. See DBOptions::row_cache for more info.