chore: add appveyor badge

Author: daviddias

README.md

@@ -19,6 +19,7 @@
   <a href="https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fipfs%2Fjs-ipfs?ref=badge_small" alt="FOSSA Status"><img src="https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fipfs%2Fjs-ipfs.svg?type=small"/></a>
   <a href="https://travis-ci.org/ipfs/js-ipfs"><img src="https://travis-ci.org/ipfs/js-ipfs.svg?branch=master" /></a>
   <a href="https://circleci.com/gh/ipfs/js-ipfs"><img src="https://circleci.com/gh/ipfs/js-ipfs.svg?style=svg" /></a>
+  <a href="https://ci.appveyor.com/project/diasdavid/js-ipfs"><img src="https://ci.appveyor.com/api/projects/status/txn75y7843r2ff0p?svg=true" /></a>
   <a href="https://coveralls.io/github/ipfs/js-ipfs?branch=master"><img src="https://coveralls.io/repos/github/ipfs/js-ipfs/badge.svg?branch=master"></a>
   <br>
   <a href="https://david-dm.org/ipfs/js-ipfs"><img src="https://david-dm.org/ipfs/js-ipfs.svg?style=flat-square" /></a>