HISTORY update for export and import column family APIs

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5587

Differential Revision: D16359919

fbshipit-source-id: cfd9c448d79a8b8e7ac1d2b661d10151df269dba

HISTORY.md

@@ -17,6 +17,7 @@
 * db_bench adds a "benchmark" stats_history, which prints out the whole stats history.
 * Rate limited deletion of WALs is only enabled if DBOptions::wal_dir is not set, or explicitly set to db_name passed to DB::Open and DBOptions::db_paths is empty, or same as db_paths[0].path
 * Overload GetAllKeyVersions() to support non-default column family.
+* Added new APIs ExportColumnFamily() and CreateColumnFamilyWithImport() to support export and import of a Column Family. https://github.com/facebook/rocksdb/issues/3469
 
 ### New Features
 * Add an option `snap_refresh_nanos` (default to 0.1s) to periodically refresh the snapshot list in compaction jobs. Assign to 0 to disable the feature.