Add an AppVeyor badge

ReadMe.md

 [![GitHub (pre-)release](https://img.shields.io/github/release/ornladios/adios2/all.svg)]()
 [![GitHub license](http://dmlc.github.io/img/apache2.svg)](./LICENSE)
-[![CircleCI](https://circleci.com/gh/ornladios/ADIOS2.svg?style=shield)](https://circleci.com/gh/ornladios/ADIOS2)
-[![Build Status](https://travis-ci.org/ornladios/adios2.svg)](https://travis-ci.org/ornladios/adios2)
 
 
+[![Circle CI](https://circleci.com/gh/ornladios/ADIOS2.svg?style=shield)](https://circleci.com/gh/ornladios/ADIOS2)
+[![Travis CI](https://travis-ci.org/ornladios/ADIOS2.svg)](https://travis-ci.org/ornladios/ADIOS2)
+[![AppVeyor CI](https://ci.appveyor.com/api/projects/status/0s2a3qp57hgbvlhj?svg=true)](https://ci.appveyor.com/project/ornladios/adios2)
+
 # Adaptable Input / Output System (ADIOS) v2.0
 This is v2.0 of the ADIOS I/O system, developed as part of the
 U.S. Department of Energy Exascale Computing Program.