Include Appveyor badge on the main page (#487)

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/google/pprof.svg?branch=master)](https://travis-ci.org/google/pprof)
+[![Build status](https://ci.appveyor.com/api/projects/status/ucu3k9t263f8syn3?svg=true)](https://ci.appveyor.com/project/gwpappveyor/pprof)
 [![codecov](https://codecov.io/gh/google/pprof/graph/badge.svg)](https://codecov.io/gh/google/pprof)
 
 # Introduction