Add AppVeyor build status badge.

Author: rillian

README.md

@@ -2,6 +2,7 @@
 
 [![Travis Build Status](https://travis-ci.org/xiph/vorbis.svg?branch=master)](https://travis-ci.org/xiph/vorbis)
 [![Jenkins Build Status](https://mf4.xiph.org/jenkins/job/libvorbis/badge/icon)](https://mf4.xiph.org/jenkins/job/libvorbis/)
+[![AppVeyor Build status](https://ci.appveyor.com/api/projects/status/github/xiph/vorbis?branch=master&svg=true)](https://ci.appveyor.com/project/rillian/vorbis)
 
 Vorbis is a general purpose audio and music encoding format
 contemporary to MPEG-4's AAC and TwinVQ, the next generation beyond