Add Alluxio to USERS.md (#5434)

Summary:
Add Alluxio's use case of RocksDB to `USERS.md` for metadata service
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5434

Differential Revision: D15766559

Pulled By: riversand963

fbshipit-source-id: b68ef851f8f92e0925c31e55296260225fdf849e

USERS.md

@@ -50,6 +50,10 @@ Check out their blog post: http://blog.cloudera.com/blog/2015/08/inside-santande
 ## Airbnb
 Airbnb is using RocksDB as a storage engine for their personalized search service. You can learn more about it here: https://www.youtube.com/watch?v=ASQ6XMtogMs
 
+## Alluxio
+[Alluxio](https://www.alluxio.io) uses RocksDB to serve and scale file system metadata to beyond 1 Billion files. The detailed design and implementation is described in this engineering blog:
+https://www.alluxio.io/blog/scalable-metadata-service-in-alluxio-storing-billions-of-files/
+
 ## Pinterest
 Pinterest's Object Retrieval System uses RocksDB for storage: https://www.youtube.com/watch?v=MtFEVEs_2Vo