add documentation on initializing Play

static/usage.html

@@ -712,6 +712,10 @@
                 features like push notifications to work properly in the background. It isn't
                 needed for the other 2 apps.</p>
 
+                <p>After installing the apps, you should open the Play Store and press sign in.
+                You don't actually need to sign in, but triggering the process will initialize the
+                apps similarly to going through Google's setup wizard process.</p>
+
                 <p>The Play Store app cannot install and update apps as it normally would since it
                 depends entirely on privileged permissions for unattended app installation,
                 updates and removal. GrapheneOS includes a partial implementation of fallback code