Adding Dgraph to list of Users (#1291)

* Adding Dgraph

* open-source

USERS.md

@@ -71,3 +71,6 @@ quasardb uses a heavily tuned RocksDB as its persistence layer.
 
 ## 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.
+
+## Dgraph
+[Dgraph](https://github.com/dgraph-io/dgraph) is an open-source, scalable, distributed, low latency, high throughput Graph database .They use RocksDB to store state locally on a machine.