Update HISTORY.md with new use of ZSTD_CDict (#4901)

Summary:
Mention feature introduced by #4849 in HISTORY.md.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4901

Differential Revision: D13746430

Pulled By: ajkr

fbshipit-source-id: f7bdea6f0522ed55428cbc521f8a9f3cd0002d4e

HISTORY.md

@@ -4,6 +4,7 @@
 * Enabled checkpoint on readonly db (DBImplReadOnly).
 * Make DB ignore dropped column families while committing results of atomic flush.
 * RocksDB may choose to preopen some files even if options.max_open_files != -1. This may make DB open slightly longer.
+* For users of dictionary compression with ZSTD v0.7.0+, we now reuse the same digested dictionary when compressing each of an SST file's data blocks for faster compression speeds.
 
 ### Public API Change
 * Transaction::GetForUpdate is extended with a do_validate parameter with default value of true. If false it skips validating the snapshot before doing the read. Similarly ::Merge, ::Put, ::Delete, and ::SingleDelete are extended with assume_tracked with default value of false. If true it indicates that call is assumed to be after a ::GetForUpdate.