Add release notes for PinnableSlice

Summary: Closes https://github.com/facebook/rocksdb/pull/2037

Differential Revision: D4822085

Pulled By: maysamyabandeh

fbshipit-source-id: 9d5a986

HISTORY.md

@@ -3,6 +3,9 @@
 ### Public API Change
 * Support dynamically change `stats_dump_period_sec` option via SetDBOptions().
 * Added ReadOptions::max_skippable_internal_keys to set a threshold to fail a request as incomplete when too many keys are being skipped when using iterators.
+* DB::Get in place of std::string accepts PinnableSlice, which avoids the extra memcpy of value to std::string in most of cases.
+    * PinnableSlice releases the pinned resources that contain the value when it is destructed or when ::Reset() is called on it.
+    * The old API that accepts std::string, although discouraged, is still supported.
 
 ### New Features
 * Memtable flush can be avoided during checkpoint creation if total log file size is smaller than a threshold specified by the user.