Add AppVeyor badge

Author: kblok

README.md

@@ -1,8 +1,10 @@
 # Puppeteer Sharp
 
 [![NuGet](https://img.shields.io/nuget/v/PuppeteerSharp.svg?style=flat-square&label=nuget&colorB=green)](NugetUrl)
+[![Build status](https://ci.appveyor.com/api/projects/status/pwfkjb0c4jfdo7lc/branch/master?svg=true)](BuildUrl)
 
 [NugetUrl]: https://www.nuget.org/packages/PuppeteerSharp/
+[BuildUrl]: https://ci.appveyor.com/project/kblok/puppeteer-sharp/branch/master
 
 Puppeteer Sharp is a .NET port of the official [Node.JS Puppeteer API](https://github.com/GoogleChrome/puppeteer).