change readme

README.md

 Pika
 ====
 
-Pika is a simple nosql database.
-Pika used leveldb as storage engine.
-The protocol used to contact with Pika is redis protocol
+Pika is redis like database.
+Pika used Nemo as storage engine, Nemo is developed base on rocksdb.
+Nemo support multi data structure by using rocksdb's kv port. Such as list,
+hash, zst
+Because rocksdb is using disk, so Nemo don't have capacity limitation.
+The protocol used to contact with Pika is redis protocol, so you don't need to
+change your code to migrate to Pika
 
 ### Thread Model
 Pika used multi threads model. The main thread accept the request and then send