Update README.md

Author: chrisbianca

README.md

@@ -21,7 +21,7 @@ npm install --save react-firebase-hooks
 yarn add react-firebase-hooks
 ```
 
-This assumes that you’re using the [npm](https://npmjs.com) package manager with a module bundler like [Webpack](https://webpack.js.org/) or [Browserify](http://browserify.org/) to consume [CommonJS](http://webpack.github.io/docs/commonjs.html) modules.
+This assumes that you’re using the [npm](https://npmjs.com) or [yarn](https://yarnpkg.com/) package managers with a module bundler like [Webpack](https://webpack.js.org/) or [Browserify](http://browserify.org/) to consume [CommonJS](http://webpack.github.io/docs/commonjs.html) modules.
 
 ## Why?