docs(readme): add appveyor badge to readme

Closes #513

Author: seanmonstar

Diff for: README.md

@@ -1,6 +1,7 @@
 # hyper
 
-[![Build Status](https://travis-ci.org/hyperium/hyper.svg?branch=master)](https://travis-ci.org/hyperium/hyper)
+[![Travis Build Status](https://travis-ci.org/hyperium/hyper.svg?branch=master)](https://travis-ci.org/hyperium/hyper)
+[![Appveyor Build status](https://ci.appveyor.com/api/projects/status/tb0n55fjs5tohdfo/branch/master?svg=true)](https://ci.appveyor.com/project/seanmonstar/hyper)
 [![Coverage Status](https://coveralls.io/repos/hyperium/hyper/badge.svg?branch=master)](https://coveralls.io/r/hyperium/hyper?branch=master)
 [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)
 [![crates.io](http://meritbadge.herokuapp.com/hyper)](https://crates.io/crates/hyper)