README.md: Add badge for AppVeyor builds (master branch)

README.md

@@ -5,6 +5,7 @@
 Rust (user-space) virtual machine for eBPF
 
 [![Build Status](https://travis-ci.org/qmonnet/rbpf.svg?branch=master)](https://travis-ci.org/qmonnet/rbpf)
+[![Build status](https://ci.appveyor.com/api/projects/status/ia74coeuhxtrcvsk/branch/master?svg=true)](https://ci.appveyor.com/project/qmonnet/rbpf/branch/master)
 [![Crates.io](https://img.shields.io/crates/v/rbpf.svg)](https://crates.io/crates/rbpf)
 
 * [Description](#description)