README: Add Appveyor badge for Windows CI results

Also updates the Travis CI badge link tests to note that it runs Linux
and macOS builds.

Author: mbland

README.md

@@ -4,7 +4,8 @@ Source: https://github.com/mbland/go-script-bash
 
 [![Latest release](https://img.shields.io/github/release/mbland/go-script-bash.svg)](https://github.com/mbland/go-script-bash/releases/latest)
 [![License](https://img.shields.io/github/license/mbland/go-script-bash.svg)](https://github.com/mbland/go-script-bash/blob/master/LICENSE.md)
-[![Continuous integration status](https://img.shields.io/travis/mbland/go-script-bash/master.svg)](https://travis-ci.org/mbland/go-script-bash)
+[![Continuous integration status for Linux and macOS](https://img.shields.io/travis/mbland/go-script-bash/master.svg)](https://travis-ci.org/mbland/go-script-bash)
+[![Continuous integration status for Windows](https://img.shields.io/appveyor/ci/mbland/go-script-bash/master.svg)](https://ci.appveyor.com/project/mbland/go-script-bash)
 [![Coverage Status](https://img.shields.io/coveralls/mbland/go-script-bash/master.svg)](https://coveralls.io/github/mbland/go-script-bash?branch=master)
 
 A `./go` script aims to abstract away many of the steps needed to develop (and