Update readme to include a note on beta packages (#1471)

README.md

@@ -30,6 +30,20 @@ npm install stripe --save
 yarn add stripe
 ```
 
+### Beta version
+
+Stripe has features in the beta phase that can be accessed via the beta version of this package.
+We would love for you to try these and share feedback with us before these features reach the stable phase.
+The beta versions can be installed in one of two ways 
+- To install the latest beta version, run the command `npm install stripe@beta --save`  
+- To install a specific beta version, replace the term "beta" in the above command with the version number like `npm install [email protected] --save` 
+
+> Note: There can be breaking changes between beta versions. Therefore we recommend pinning the package version to a specific beta version in your package.json file. This way you can install the same version each time without breaking changes unless you are intentionally looking for the latest beta version. 
+
+We highly recommend keeping an eye on when the beta feature you are interested in goes from beta to stable so that you can move from using a beta version of the SDK to the stable version.  
+
+The versions tab on the [stripe page on npm](https://www.npmjs.com/package/stripe) lists the current tags in use. The `beta` tag here corresponds to the the latest beta version of the package.
+
 ## Usage
 
 The package needs to be configured with your account's secret key, which is