Document trials without a payment method

trial_payment_method (TrialPaymentMethod::REQUIRED, the default, or
::OPTIONAL) on app subscriptions, subscriptions and checkout sessions:
an optional trial can be confirmed without a payment method and is
cancelled at its end if there is still none (cancellation_reason and
subscription.cancelled data.reason
CancellationReason::TRIAL_ENDED_WITHOUT_PAYMENT_METHOD).
subscription.trial_will_end carries data.payment_method_missing.

src/Id/Enums/CancellationReason.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Anikeen\Id\Enums;
+
+/**
+ * `cancellation_reason` of a subscription (and `data.reason` of `subscription.cancelled`): why
+ * Anikeen ID's billing ended it. Null when the user, the app or an admin cancelled it.
+ */
+class CancellationReason
+{
+    /** Its free trial (`trial_payment_method` optional) ended without a payment method: cancelled at the trial's end, nothing billed. */
+    const TRIAL_ENDED_WITHOUT_PAYMENT_METHOD = 'trial_ended_without_payment_method';
+}