update README with crates.io badge

Author: andygrove

README.md

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 # mysql-proxy-rs
 
+[![Version](https://img.shields.io/crates/v/mysql-proxy.svg)](https://crates.io/crates/mysql-proxy)
+
 An implementation of a MySQL proxy server built on top of `tokio-core`. Tested with `rustc 1.13.0-nightly (cbe4de78e 2016-09-05)`.
 
 ## Usage