Add a badge for AppVeyor build.

Change-Id: Ic59d78d699af0e5f50377f83c8a7ea9a78074d81

README.md

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 Protocol Buffers - Google's data interchange format
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-[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf)
+[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf) [![Build status](https://ci.appveyor.com/api/projects/status/73ctee6ua4w2ruin?svg=true)](https://ci.appveyor.com/project/protobuf/protobuf)
 
 Copyright 2008 Google Inc.