Update HISTORY.md to include the Public API Change for DB::OpenForReadonly introduced earlier (#7023)

Summary:
`DB::OpenForReadOnly()` now returns `Status::NotFound` when the specified DB directory does not exist. Previously the error returned depended on the underlying `Env`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7023

Reviewed By: ajkr

Differential Revision: D22207845

Pulled By: gg814

fbshipit-source-id: f35830811a0e67efb0ee82eda3a9739bc526baba

HISTORY.md

@@ -12,6 +12,7 @@
 
 ### Public API Change
 * `DB::GetDbSessionId(std::string& session_id)` is added. `session_id` stores a unique identifier that gets reset every time the DB is opened. This DB session ID should be unique among all open DB instances on all hosts, and should be unique among re-openings of the same or other DBs. This identifier is recorded in the LOG file on the line starting with "DB Session ID:".
+* `DB::OpenForReadOnly()` now returns `Status::NotFound` when the specified DB directory does not exist. Previously the error returned depended on the underlying `Env`. This change is available in all 6.11 releases as well.
 
 ### New Features
 * DB identity (`db_id`) and DB session identity (`db_session_id`) are added to table properties and stored in SST files. SST files generated from SstFileWriter and Repairer have DB identity “SST Writer” and “DB Repairer”, respectively. Their DB session IDs are generated in the same way as `DB::GetDbSessionId`. The session ID for SstFileWriter (resp., Repairer) resets every time `SstFileWriter::Open` (resp., `Repairer::Run`) is called.