chore: Add Firefox Add-ons badge to README.md

README.md

@@ -2,6 +2,7 @@ Violentmonkey
 =============
 
 [![Chrome Web Store](https://img.shields.io/chrome-web-store/v/jinjaccalgkegednnccohejagnlnfdag.svg)](https://chrome.google.com/webstore/detail/violentmonkey/jinjaccalgkegednnccohejagnlnfdag)
+[![Firefox Add-ons](https://img.shields.io/amo/v/violentmonkey.svg)](https://addons.mozilla.org/firefox/addon/violentmonkey)
 [![Gitter](https://img.shields.io/gitter/room/violentmonkey/violentmonkey.svg)](https://gitter.im/violentmonkey/violentmonkey)
 
 Violentmonkey provides userscripts support for browsers.