Add inbound mail handling

Add normalized event, signal, and webhooks for inbound mail.

Closes #43
Closes #86

docs/esps/sendgrid.rst

@@ -302,6 +302,37 @@ for each event in the batch.)
 .. _Sendgrid event: https://sendgrid.com/docs/API_Reference/Webhooks/event.html
 
 
+.. _sendgrid-inbound:
+
+Inbound webhook
+---------------
+
+If you want to receive email from SendGrid through Anymail's normalized :ref:`inbound <inbound>`
+handling, follow SendGrid's `Inbound Parse Webhook`_ guide to set up
+Anymail's inbound webhook.
+
+The Destination URL setting will be:
+
+   :samp:`https://{random}:{random}@{yoursite.example.com}/anymail/sendgrid/inbound/`
+
+     * *random:random* is an :setting:`ANYMAIL_WEBHOOK_AUTHORIZATION` shared secret
+     * *yoursite.example.com* is your Django site
+
+Be sure the URL has a trailing slash. (SendGrid's inbound processing won't follow Django's
+:setting:`APPEND_SLASH` redirect.)
+
+If you want to use Anymail's normalized :attr:`~anymail.inbound.AnymailInboundMessage.spam_detected` and
+:attr:`~anymail.inbound.AnymailInboundMessage.spam_score` attributes, be sure to enable the "Check
+incoming emails for spam" checkbox.
+
+You have a choice for SendGrid's "POST the raw, full MIME message" checkbox. Anymail will handle
+either option (and you can change it at any time). Enabling raw MIME will give the most accurate
+representation of *any* received email (including complex forms like multi-message mailing list
+digests). But disabling it *may* use less memory while processing messages with many large attachments.
+
+.. _Inbound Parse Webhook:
+   https://sendgrid.com/docs/Classroom/Basics/Inbound_Parse_Webhook/setting_up_the_inbound_parse_webhook.html
+
 
 .. _sendgrid-v3-upgrade: