Event-tracking webhooks

Closes #3

docs/esps/postmark.rst

@@ -113,3 +113,37 @@ see :ref:`unsupported-features`.
 
 **No delayed sending**
   Postmark does not support :attr:`~anymail.message.AnymailMessage.send_at`.
+
+
+
+.. _postmark-webhooks:
+
+Status tracking webhooks
+------------------------
+
+If you are using Anymail's normalized :ref:`status tracking <event-tracking>`, enter
+the url in your `Postmark account settings`_, under Servers > *your server name* >
+Settings > Outbound > Webhooks. You should enter this same Anymail tracking URL
+for both the "Bounce webhook" and "Opens webhook" (if you want to receive both
+types of events):
+
+   :samp:`https://{random}:{random}@{yoursite.example.com}/anymail/postmark/tracking/`
+
+     * *random:random* is an :setting:`ANYMAIL_WEBHOOK_AUTHORIZATION` shared secret
+     * *yoursite.example.com* is your Django site
+
+Anymail doesn't care about the "include bounce content" and "post only on first open"
+Postmark webhook settings: whether to use them is your choice.
+
+If you use multiple Postmark servers, you'll need to repeat entering the webhook
+settings for each of them.
+
+Postmark will report these Anymail :attr:`~anymail.signals.AnymailTrackingEvent.event_type`\s:
+rejected, failed, bounced, deferred, autoresponded, opened, complained, unsubscribed, subscribed.
+(Postmark does not support sent, delivered, or clicked events.)
+
+The event's :attr:`~anymail.signals.AnymailTrackingEvent.esp_event` field will be
+a `dict` of Postmark `bounce <http://developer.postmarkapp.com/developer-bounce-webhook.html>`_
+or `open <http://developer.postmarkapp.com/developer-open-webhook.html>`_ webhook data.
+
+.. _Postmark account settings: https://account.postmarkapp.com/servers