README: Add mongodb to backend list

README.rst

@@ -57,8 +57,8 @@ Features
 
     * When a task has been executed, the return value can be stored using
       either a MySQL/Oracle/PostgreSQL/SQLite database, Memcached,
-      or Tokyo Tyrant back-end. For high-performance you can also use
-      AMQP to publish results.
+      `MongoDB`_ or `Tokyo Tyrant`_ back-end. For high-performance you can
+      also use AMQP messages to publish results.
 
     * If the task raises an exception, the exception instance is stored,
       instead of the return value.
@@ -95,6 +95,9 @@ Features
     * Can be configured to send e-mails to the administrators when a task
       fails.
 
+.. _`MongoDB`: http://www.mongodb.org/
+.. _`Tokyo Tyrant`: http://tokyocabinet.sourceforge.net/
+
 API Reference Documentation
 ===========================