Add USERS.md

Summary: See the file.

Test Plan: none

Reviewers: lgalanis, meyering, MarkCallaghan, yhchiang, rven, anthony, kradhakrishnan, jayadev, sdong

Reviewed By: jayadev

Subscribers: jayadev, rdallman, andybons, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36621

USERS.md

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+This document lists users of RocksDB and their use cases. If you are using RocksDB, please open a pull request and add yourself to the list.
+
+## Facebook
+At Facebook, we use RocksDB as a backend for many different stateful services. We're also experimenting with running RocksDB as a storage engine for two databases:
+1. MyRocks -- https://github.com/MySQLOnRocksDB/mysql-5.6
+2. MongoRocks -- https://github.com/mongodb-partners/mongo-rocks
+
+## Yahoo
+Yahoo is using RocksDB as a storage engine for their biggest distributed data store Sherpa.
+
+## CockroachDB
+CockroachDB is an open-source geo-replicated transactional database (still in development). They are using RocksDB as their storage engine. Check out their github: https://github.com/cockroachdb/cockroach
+
+## DNANexus
+DNANexus is using RocksDB to speed up processing of genomics data.
+You can learn more from this great blog post by Mike Lin: http://devblog.dnanexus.com/faster-bam-sorting-with-samtools-and-rocksdb/
+
+## Iron.io
+Iron.io is using RocksDB as a storage engine for their distributed queueing system.
+Learn more from Tech Talk by Reed Allman: http://www.youtube.com/watch?v=HTjt6oj-RL4
+
+