Merge pull request #166 from okv/patch-2

Add link to mongodb store to store section at readme

README.md

@@ -221,6 +221,8 @@ Available data stores are:
 - MemoryStore: _(default)_ Simple in-memory option. Does not share state when app has multiple processes or servers.
 - [rate-limit-redis](https://npmjs.com/package/rate-limit-redis): A [Redis](http://redis.io/)-backed store, more suitable for large or demanding deployments.
 - [rate-limit-memcached](https://npmjs.org/package/rate-limit-memcached): A [Memcached](https://memcached.org/)-backed store.
+- [rate-limit-mongo](https://www.npmjs.com/package/rate-limit-mongo): A [MongoDB](https://www.mongodb.com/)-backed store.
+
 
 You may also create your own store. It must implement the following in order to function: