Added Postgres-XL

http://www.postgres-xl.org/

README.md

@@ -271,6 +271,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [MySQL](http://dev.mysql.com/) - Most popular RDBMS server.
 * [Percona Server](http://www.percona.com/software) - Enhanced, drop-in MySQL replacement.
 * [PostgreSQL](http://www.postgresql.org/) - Object-relational database management system (ORDBMS).
+* [PostgreSQL-XL](http://www.postgres-xl.org/) - Scalable Open Source PostgreSQL-based database cluster.
 * [SQLite](http://sqlite.org/) - Library that implements a self-contained, serverless, zero-configuration, transactional SQL DBS.
 
 ## SMTP