doc: Add AppVeyor status badge to README

[ci skip]
diff --git a/README.md b/README.md
index 3376211..6528b87 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/gflags/gflags.svg?branch=master)](https://travis-ci.org/gflags/gflags)
+[![Build status](https://ci.appveyor.com/api/projects/status/4ctod566ysraus74/branch/master?svg=true)](https://ci.appveyor.com/project/schuhschuh/gflags/branch/master)
 
 24 March 2015
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