Add AppVeyor CI link to the README

Author: sodabrew

README.md

@@ -1,6 +1,7 @@
 # Mysql2 - A modern, simple and very fast MySQL library for Ruby - binding to libmysql
 
-[![Build Status](https://travis-ci.org/brianmario/mysql2.png)](https://travis-ci.org/brianmario/mysql2)
+Travis CI [![Travis CI Status](https://travis-ci.org/brianmario/mysql2.png)](https://travis-ci.org/brianmario/mysql2)
+Appveyor CI [![Appveyor CI Status](https://ci.appveyor.com/api/projects/status/github/sodabrew/mysql2)](https://ci.appveyor.com/project/sodabrew/mysql2)
 
 The Mysql2 gem is meant to serve the extremely common use-case of connecting, querying and iterating on results.
 Some database libraries out there serve as direct 1:1 mappings of the already complex C APIs available.