Replace TeamCity badge with AppVeyor badge

Author: Daniel15

README.md

@@ -5,6 +5,7 @@ ReactJS.NET is a library that makes it easier to use Facebook's
 [React](http://facebook.github.io/react/) and
 [JSX](http://facebook.github.io/react/docs/jsx-in-depth.html) from C#.
 
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 [![Build status](http://img.shields.io/teamcity/codebetter/ReactJSNet_Master.svg)](http://teamcity.codebetter.com/viewType.html?buildTypeId=ReactJSNet_Master&guest=1) 
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