Update readme with appveyor build status

Meaghan Lewis · web-flow · commit 255343f2c420 · 2017-11-08T15:30:29.000-08:00
diff --git a/README.md b/README.md
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 # GitHub Extension for Visual Studio
 
+[![Build status](https://ci.appveyor.com/api/projects/status/dl8is5iqwt9qf3t7/branch/master?svg=true)](https://ci.appveyor.com/project/github-windows/visualstudio/branch/master)
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 ## Notices
 
 ### VS 2017 v15.3 users, please read
