Add Apache Spark as a user (#10993)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/10993

Reviewed By: ajkr

Differential Revision: D41543962

Pulled By: cbi42

fbshipit-source-id: a895d7863543bd64734c5c9faa7b55b0732b3d60

USERS.md

@@ -79,6 +79,9 @@ quasardb uses a heavily tuned RocksDB as its persistence layer.
 ## TiKV
 [TiKV](https://github.com/pingcap/tikv) is a GEO-replicated, high-performance, distributed, transactional key-value database. TiKV is powered by Rust and Raft. TiKV uses RocksDB as its persistence layer.
 
+## Apache Spark
+[Spark Structured Streaming](https://docs.databricks.com/structured-streaming/rocksdb-state-store.html) uses RocksDB as the local state store.
+
 ## Apache Flink
 [Apache Flink](https://flink.apache.org/news/2016/03/08/release-1.0.0.html) uses RocksDB to store state locally on a machine.