Update HISTORY.md for #8428 (#9001)

Summary:
Context:
HISTORY.md was not properly updated along with the change in https://github.com/facebook/rocksdb/pull/8428, where we introduced a change of accounting compression dictionary buffering memory and an extra condition of triggering data unbuffering.
Updated HISTORY.md for https://github.com/facebook/rocksdb/pull/8428 in 6.25.0 HISTORY.md section.
Updated blog post https://rocksdb.org/blog/2021/05/31/dictionary-compression.html.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9001

Reviewed By: ajkr

Differential Revision: D31517836

Pulled By: hx235

fbshipit-source-id: 01f6b30de4e1ff6b315aa8221139d9b700c7c629

HISTORY.md

@@ -46,6 +46,7 @@
 * Batch blob read requests for `DB::MultiGet` using `MultiRead`.
 * Add support for fallback to local compaction, the user can return `CompactionServiceJobStatus::kUseLocal` to instruct RocksDB to run the compaction locally instead of waiting for the remote compaction result.
 * Add built-in rate limiter's implementation of `RateLimiter::GetTotalPendingRequest(int64_t* total_pending_requests, const Env::IOPriority pri)` for the total number of requests that are pending for bytes in the rate limiter.
+* Charge memory usage during data buffering, from which training samples are gathered for dictionary compression, to block cache. Unbuffering data can now be triggered if the block cache becomes full and `strict_capacity_limit=true` for the block cache, in addition to existing conditions that can trigger unbuffering.
 
 ### Public API change
 * Remove obsolete implementation details FullKey and ParseFullKey from public API