Update USERS.md add user Pika

Summary:
add rocksdb users pika
Closes https://github.com/facebook/rocksdb/pull/1805

Differential Revision: D4462809

Pulled By: siying

fbshipit-source-id: 7289886

USERS.md

@@ -77,3 +77,6 @@ quasardb uses a heavily tuned RocksDB as its persistence layer.
 
 ## Uber
 [Uber](http://eng.uber.com/cherami/) uses RocksDB as a durable and and scalable task queue.
+
+## 360 Pika
+[360](http://www.360.cn/) [Pika](https://github.com/Qihoo360/pika) is a nosql compatible with redis. With the huge amount of data stored, redis may suffer for a capacity bottleneck, and pika was born for solving it. It has widely been widely used in many company