Update README.md

Added Varnish cache.

README.md

@@ -323,6 +323,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [Lighttpd](http://www.lighttpd.net/) -  Web server more optimized for speed-critical environments.
 * [Nginx](http://nginx.org/) -  Reverse proxy, load balancer, HTTP cache, and web server.
 * [uWSGI](https://github.com/unbit/uwsgi/) - The uWSGI project aims at developing a full stack for building hosting services.
+* [Varnish](https://www.varnish-cache.org/) - Very popular HTTP cache server.
 
 ## Wikis
 *Wiki software.*