Add Solana's RocksDB use case in USERS.md (#9558)

Summary:
Add Solana's RocksDB use case in USERS.md.

Solana is a fast, secure, scalable, and decentralized blockchain.  It uses RocksDB as the underlying storage for its ledger store.

github: https://github.com/solana-labs/solana

Pull Request resolved: facebook/rocksdb#9558

Reviewed By: jay-zhuang

Differential Revision: D34249087

Pulled By: riversand963

fbshipit-source-id: 7524eff4952e2676e8520ac491ffb6a686fb4d7e

USERS.md

@@ -121,6 +121,8 @@ LzLabs is using RocksDB as a storage engine in their multi-database distributed
 ## Kafka
 [Kafka](https://kafka.apache.org/) is an open-source distributed event streaming platform, it uses RocksDB to store state in Kafka Streams: https://www.confluent.io/blog/how-to-tune-rocksdb-kafka-streams-state-stores-performance/.
 
+## Solana Labs
+[Solana](https://github.com/solana-labs/solana) is a fast, secure, scalable, and decentralized blockchain.  It uses RocksDB as the underlying storage for its ledger store.
+
 ## Others
 More databases using RocksDB can be found at [dbdb.io](https://dbdb.io/browse?embeds=rocksdb).
-