Include documentation in the application

doc/postmark.markdown

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+Postmark
+========
+
+You can use the service [Postmark](https://postmarkapp.com/) to create tasks directly by email.
+
+This integration works with the inbound email service of Postmark.
+Kanboard use a webhook to handle incoming emails.
+
+The [incoming email workflow is described here](create-tasks-by-email.markdown).
+
+Postmark configuration
+----------------------
+
+Just follow the [official documentation about inbound email processing](http://developer.postmarkapp.com/developer-process-configure.html).
+Basically, you have to forward your own domain or subdomain to a specific Postmark email address.
+
+The Kanboard webhook url is displayed in **Settings > Integrations > Postmark**
+
+Kanboard configuration
+----------------------
+
+1. Be sure that your users have an email address in their profiles
+2. Assign a project identifier to the desired projects: **Project settings > Edit**
+3. Try to send an email to your project
+
+Troubleshootings
+----------------
+
+- Test the webhook url from the Postmark console, you should have a status code `200 OK`