Add Extension.js (#87)

readme.md

@@ -64,6 +64,7 @@ Apps that help you manage your extensions.
 - [webextension-manifest-loader](https://github.com/jsmnbom/webextension-manifest-loader) - Webpack loader that loads browser tailored manifest.json. It also imports all importable properties, allowing you to have 'manifest.json' as your only webpack entry point.
 - [webpack-extension-reloader](https://github.com/rubenspgcavalcante/webpack-extension-reloader) - A Webpack plugin to automatically reload browser extensions during development.
 - [webpack-target-webextension](https://github.com/awesome-webextension/webpack-target-webextension) - Adds code-splitting support to WebExtensions build with Webpack.
+- [Extension.js](https://github.com/cezaraugusto/extension.js) - Plug-and-play, zero-config, cross-browser extension development tool.
 
 ## Testing