Add draft of payment api documentation

doc/development/api/payment.rst

@@ -0,0 +1,28 @@
+.. highlight:: python
+   :linenothreshold: 5
+
+Writing a payment provider plugin
+=================================
+
+In this document, we will walk through the creation of a payment provider plugin.
+
+Please read :ref:`Creating a plugin <pluginsetup>` first, if you haven't already.
+
+The signal
+----------
+
+The payment provider API does not make a lot of usage from signals, however, it
+does use a signal to get a list of all available payment providers. Your plugin
+should listen for this signal and return the subclass of ``pretix.base.payment.PaymentProvider``
+that we'll soon create::
+
+    from django.dispatch import receiver
+
+    from pretix.base.signals import register_payment_providers
+
+    from .payment import BankTransfer
+
+
+    @receiver(register_payment_providers)
+    def register_payment_provider(sender, **kwargs):
+        return BankTransfer