Add AppVeyor badge to README.md
Add AppVeyor badge to README.md

Now that we have AppVeyor builds running, let's show them off!

Adapted from the instructions here:
https://www.appveyor.com/docs/status-badges/#badges-for-projects-with-public-repositories-on-github-and-bitbucket

josephfrazier/browser-extension,josephfrazier/browser-extension,OctoLinker/browser-extension,OctoLinker/browser-extension,josephfrazier/browser-extension,OctoLinker/browser-extension