Update HISTORY.md (#7887)

Summary:
Mention the forward compatibility fix for WAL related version edits.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7887

Reviewed By: ltamasi

Differential Revision: D25982494

Pulled By: cheng-chang

fbshipit-source-id: 4be292aa4bf7fbc8a27c0bef1e7a98ad3ea8e1fa

HISTORY.md

@@ -4,6 +4,9 @@
 * When verifying full file checksum with `DB::VerifyFileChecksums()`, we now fail with `Status::InvalidArgument` if the name of the checksum generator used for verification does not match the name of the checksum generator used for protecting the file when it was created.
 * Since RocksDB does not continue write the same file if a file write fails for any reason, the file scope write IO error is treated the same as retryable IO error. More information about error handling of file scope IO error is included in `ErrorHandler::SetBGError`.
 
+### Bug Fixes
+* Version older than 6.15 cannot decode VersionEdits `WalAddition` and `WalDeletion`, fixed this by changing the encoded format of them to be ignorable by older versions.
+
 ### Public API Change
 * Add a public API WriteBufferManager::dummy_entries_in_cache_usage() which reports the size of dummy entries stored in cache (passed to WriteBufferManager). Dummy entries are used to account for DataBlocks.