Add leveldb cache to list of available backends

Author: gregjones

README.md

@@ -18,3 +18,4 @@ Cache backends
 - https://github.com/gregjones/httpcache/diskcache provides a filesystem-backed cache using the [diskv](https://github.com/peterbourgon/diskv) library.
 - https://github.com/gregjones/httpcache/memcache provides memcache implementations, for both App Engine and 'normal' memcache servers
 - https://github.com/sourcegraph/s3cache uses Amazon S3 for storage.
+- https://github.com/gregjones/httpcache/leveldbcache provides a filesystem-backed cache using [leveldb](https://github.com/syndtr/goleveldb/leveldb)