Update README.md

add appveyor badge

Author: AzothAmmo

README.md

@@ -78,6 +78,7 @@ cereal is licensed under the [BSD license](http://opensource.org/licenses/BSD-3-
 ## cereal build status
 
 * develop : [![Build Status](https://travis-ci.org/USCiLab/cereal.png?branch=develop)](https://travis-ci.org/USCiLab/cereal)
+[![Build status](https://ci.appveyor.com/api/projects/status/91aou6smj36or0vb/branch/develop?svg=true)](https://ci.appveyor.com/project/AzothAmmo/cereal/branch/develop)
 
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