update readme with notice about new driver

Author: saghm

README.md

@@ -6,6 +6,8 @@
 MongoDB Rust Driver Prototype
 =============================
 
+**NOTE**: This driver is superseded by the [official MongoDB Rust driver](https://github.com/mongodb/mongo-rust-driver), and will no longer be updated.
+
 This branch contains active development on a new driver written for Rust 1.x and MongoDB 3.0.x.
 
 The API and implementation are currently subject to change at any time. You should not use this driver in production as it is still under development and is in no way supported by MongoDB Inc. We absolutely encourage you to experiment with it and provide us feedback on the API, design, and implementation. Bug reports and suggestions for improvements are welcomed, as are pull requests.