Update README for subscribing

This change updates a long-standing piece of Push Subscription
directions in the README which demonstrates setting of the
applicationServerKey into a Uint8Array from the decoded raw bytes of the
vapid public key.

This step is cumbersome and there is a simpler alternative.

The Push API docs for the PushSubscriptionOptions interface say of the
applicationServerKey:

> When provided as a DOMString, the value MUST be encoded using the
base64url encoding [RFC7515].
https://www.w3.org/TR/push-api/#dom-pushsubscriptionoptions-applicationserverkey

RFC 7515 states that the Base64 url-safe encoding omit the padding
characters:

> Base64 encoding using the URL- and filename-safe character set defined
in Section 5 of RFC 4648 [RFC4648], with all trailing '=' characters
omitted https://www.rfc-editor.org/rfc/rfc7515

It turns out the generated vapid public is already Base64 url-safe
encoded! However, for historical reasons, Ruby's Base64.urlsafe_encode64
method includes the padding character, "=", incorrectly by default. This
behavior exists in the library's current vapid key generation.
Therefore, the directions include explicit deletion of the "=" character.

For historical discussion of Ruby's encoding issue, see:
https://bugs.ruby-lang.org/issues/10740

README.md

@@ -58,7 +58,7 @@ navigator.serviceWorker.register('/service-worker.js')
 The VAPID public key you generated earlier is made available to the client as a `UInt8Array`. To do this, one way would be to expose the urlsafe-decoded bytes from Ruby to JavaScript when rendering the HTML template.
 
 ```javascript
-window.vapidPublicKey = new Uint8Array(<%= Base64.urlsafe_decode64(ENV['VAPID_PUBLIC_KEY']).bytes %>);
+window.vapidPublicKey = <%= ENV['VAPID_PUBLIC_KEY'].delete('=') %>);
 ```
 
 Your JavaScript code uses the `pushManager` interface to subscribe to push notifications, passing the VAPID public key to the subscription settings.