Add build badge to readme

Author: willsmythe

README.md

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 # Auth samples for Team Services
 
+![status](https://mseng.visualstudio.com/_apis/public/build/definitions/b924d696-3eae-4116-8443-9a18392d8544/5326/badge)
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 Samples that show how to auth with Visual Studio Team Services and Team Foundation Server.
 
 Learn more about [integrating with Team Services](https://www.visualstudio.com/docs/integrate/extensions/overview)