Update HISTORY.md for new rate limiter io priorities (#8896)

Summary: Pull Request resolved: #8896

Reviewed By: ajkr

Differential Revision: D30846120

Pulled By: hx235

fbshipit-source-id: 9224ebce5437d63b0fb8af9171c6041a9ea5d90f

HISTORY.md

@@ -15,6 +15,7 @@
 * Added a ticker statistic, "rocksdb.verify_checksum.read.bytes", reporting how many bytes were read from file to serve `VerifyChecksum()` and `VerifyFileChecksums()` queries.
 * Added ticker statistics, "rocksdb.backup.read.bytes" and "rocksdb.backup.write.bytes", reporting how many bytes were read and written during backup.
 * Added properties for BlobDB: `rocksdb.num-blob-files`, `rocksdb.blob-stats`, `rocksdb.total-blob-file-size`, and `rocksdb.live-blob-file-size`. The existing property `rocksdb.estimate_live-data-size` was also extended to include live bytes residing in blob files.
+* Added two new RateLimiter IOPriorities: `Env::IO_USER`,`Env::IO_MID`. `Env::IO_USER` will have superior priority over all other RateLimiter IOPriorities without being subject to fair scheduling constraint.
 
 ### Public API change
 * Remove obsolete implementation details FullKey and ParseFullKey from public API