Added a note that One-Tap beta test program is currently closed. (#567)

This works locally but currently can't be used in production unless you are already on the whitelist. I think it's important that people know this before developing and testing with this feature.

Author: jake-nz

README.md

@@ -449,6 +449,8 @@ non HTTP/HTTPS environments.
 
 #### One-tap sign-up
 
+> **Note:** The beta test program for this API is currently closed.  
+
 [One-tap sign-up](https://developers.google.com/identity/one-tap/web/overview)
 provides seamless authentication flows to
 your users with Google's one tap sign-up and automatic sign-in APIs.