Update README.md

integrated appveyor into readme.md

Author: Stefan Seeland

README.md

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+[![Build status](https://ci.appveyor.com/api/projects/status/xmx1t9ieuxqjeeh8?svg=true)](https://ci.appveyor.com/project/stesee/mencoder-sharp)
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 # mencoder-sharp
 Automatically exported from code.google.com/p/mencoder-sharp