CONTRIBUTING: Add Appveyor badge for Windows CI

Also updates the Travis CI badge link to note that it runs Linux and
macOS builds. Should've been part of the previous commit that updated
`README.md`.

Author: mbland

CONTRIBUTING.md

@@ -146,7 +146,8 @@ repository and issuing pull requests to the original.
 
 ## Testing
 
-- Continuous integration status: [![Continuous integration status](https://travis-ci.org/mbland/go-script-bash.png?branch=master)](https://travis-ci.org/mbland/go-script-bash)
+- Continuous integration status for Linux and macOS: [![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: [![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: [![Coverage Status](https://coveralls.io/repos/github/mbland/go-script-bash/badge.svg?branch=master)](https://coveralls.io/github/mbland/go-script-bash?branch=master)
 
 No bug fixes or new features will be accepted without accompanying tests.