Update README.md to include AppVeyor build status.

README.md

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 yuzu emulator
 =============
 [![Travis CI Build Status](https://travis-ci.org/yuzu-emu/yuzu.svg?branch=master)](https://travis-ci.org/yuzu-emu/yuzu)
+[![AppVeyor CI Build Status](https://ci.appveyor.com/api/projects/status/77k97svb2usreu68?svg=true)](https://ci.appveyor.com/project/bunnei/yuzu)
 
 yuzu is an experimental open-source emulator for the Nintendo Switch from the creators of [Citra](https://citra-emu.org/).