add sample for custom access token script

Author: lakhansamani

.env.sample

@@ -8,4 +8,5 @@ JWT_TYPE=HS256
 ROLES=user
 DEFAULT_ROLES=user
 PROTECTED_ROLES=admin
-JWT_ROLE_CLAIM=role
+JWT_ROLE_CLAIM=role
+function(user,tokenPayload){var data = tokenPayload;data.extra = {'x-extra-id': user.id};return data;}