add direct I/O to version notes 5.2.0

Summary:
let users know this feature is ready
Closes https://github.com/facebook/rocksdb/pull/1915

Differential Revision: D4610842

Pulled By: lightmark

fbshipit-source-id: d102772

HISTORY.md

@@ -10,16 +10,19 @@
 ### Public API Change
 * NewLRUCache() will determine number of shard bits automatically based on capacity, if the user doesn't pass one. This also impacts the default block cache when the user doesn't explict provide one.
 * Change the default of delayed slowdown value to 16MB/s and further increase the L0 stop condition to 36 files.
+* Options::use_direct_writes and Options::use_direct_reads are now ready to use.
 * (Experimental) Two-level indexing that partition the index and creates a 2nd level index on the partitions. The feature can be enabled by setting kTwoLevelIndexSearch as IndexType and configuring index_per_partition.
 
 ### New Features
 * Added new overloaded function GetApproximateSizes that allows to specify if memtable stats should be computed only without computing SST files' stats approximations.
 * Added new function GetApproximateMemTableStats that approximates both number of records and size of memtables.
+* Add Direct I/O mode for SST file I/O
 
 ### Bug Fixes
 * RangeSync() should work if ROCKSDB_FALLOCATE_PRESENT is not set
 * Fix wrong results in a data race case in Get()
 * Some fixes related to 2PC.
+* Fix bugs of data corruption in direct I/O
 
 ## 5.1.0 (01/13/2017)
 * Support dynamically change `delete_obsolete_files_period_micros` option via SetDBOptions().