Add first-availability note to README (#989)

* Add first-availability note to README

* rename fa to beta

README.md

@@ -8,6 +8,8 @@ Auth0 SDK for Single Page Applications using [Authorization Code Grant Flow with
 ![Downloads](https://img.shields.io/npm/dw/@auth0/auth0-spa-js)
 [![License](https://img.shields.io/:license-mit-blue.svg?style=flat)](https://opensource.org/licenses/MIT)
 
+> ℹ️ A new major version of Auth0-SPA-JS is available in **Beta**! Try it out today, any feedback is appreciated. Read all about it on [the `beta` branch](https://github.com/auth0/auth0-spa-js/tree/beta).
+
 ## Table of Contents
 
 - [Documentation](#documentation)