Mention sst_dump cmd=recompress changes (#5807)

Summary:
As requested by siying in https://github.com/facebook/rocksdb/pull/5791#issuecomment-531417468
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5807

Differential Revision: D17399349

fbshipit-source-id: 5986c3894f356becd393fee0f1aeadcd9affc798

HISTORY.md

@@ -10,6 +10,7 @@
 * VerifyChecksum() by default will issue readahead. Allow ReadOptions to be passed in to those functions to override the readhead size. For checksum verifying before external SST file ingestion, a new option IngestExternalFileOptions.verify_checksums_readahead_size, is added for this readahead setting.
 * When user uses options.force_consistency_check in RocksDb, instead of crashing the process, we now pass the error back to the users without killing the process.
 * Add an option `memtable_insert_hint_per_batch` to WriteOptions. If it is true, each WriteBatch will maintain its own insert hints for each memtable in concurrent write. See include/rocksdb/options.h for more details.
+* The `sst_dump` command line tool `recompress` command now displays how many blocks were compressed and how many were not, in particular how many were not compressed because the compression ratio was not met (12.5% threshold for GoodCompressionRatio), as seen in the `number.block.not_compressed` counter stat since version 6.0.0.
 ### Public API Change
 * Added max_write_buffer_size_to_maintain option to better control memory usage of immutable memtables.
 * Added a lightweight API GetCurrentWalFile() to get last live WAL filename and size. Meant to be used as a helper for backup/restore tooling in a larger ecosystem such as MySQL with a MyRocks storage engine.