Docs: add ESP feature support matrix

[ci skip]

Author: medmunds

docs/esps/index.rst

@@ -1,10 +1,11 @@
 .. _supported-esps:
 
 Supported ESPs
---------------
+==============
 
-Anymail supports these ESPs. Click in for the specific
-settings required and notes about any quirks or limitations:
+Anymail currently supports these Email Service Providers.
+Click an ESP's name for specific Anymail settings required,
+and notes about any quirks or limitations:
 
 .. these are listed in alphabetical order
 
@@ -17,7 +18,44 @@ settings required and notes about any quirks or limitations:
    sendgrid
 
 
-Don't see your favorite ESP here? You can suggest that
-Anymail add it, or even :ref:`contribute <contributing>`
-your own implementation to Anymail.
+Anymail feature support
+-----------------------
 
+The table below summarizes the Anymail features supported for each ESP.
+
+.. currentmodule:: anymail.message
+
+======================================    ====================== ======================= ======================= =======================
+Email Service Provider                    :ref:`mailgun-backend` :ref:`mandrill-backend` :ref:`postmark-backend` :ref:`sendgrid-backend`
+======================================    ====================== ======================= ======================= =======================
+.. rubric:: :ref:`Anymail send options <anymail-send-options>`
+----------------------------------------------------------------------------------------------------------------------------------------
+:attr:`~AnymailMessage.metadata`          Yes                    Yes                     No                      Yes
+:attr:`~AnymailMessage.send_at`           Yes                    Yes                     No                      Yes
+:attr:`~AnymailMessage.tags`              Yes                    Yes                     Max 1 tag               Yes
+:attr:`~AnymailMessage.track_clicks`      Yes                    Yes                     No                      Yes
+:attr:`~AnymailMessage.track_opens`       Yes                    Yes                     Yes                     Yes
+
+.. rubric:: :ref:`Status <esp-send-status>` and tracking
+----------------------------------------------------------------------------------------------------------------------------------------
+:attr:`~AnymailMessage.anymail_status`    Yes                    Yes                     Yes                     Yes
+======================================    ====================== ======================= ======================= =======================
+
+
+.. Status tracking webhooks                  (coming)               (coming)                (coming)                (coming)
+.. .. rubric:: :ref:`inbound`
+.. ----------------------------------------------------------------------------------------------------------------------------------------
+.. Inbound webhooks                          (coming)               (coming)                (coming)                (coming)
+
+Trying to choose an ESP? Please **don't** start with this table. It's far more
+important to consider things like an ESP's deliverability stats, latency, uptime,
+and support for developers. The *number* of extra features an ESP offers is almost
+meaningless. (And even specific features don't matter if you don't plan to use them.)
+
+
+Other ESPs
+----------
+
+Don't see your favorite ESP here? Anymail is designed to be extensible.
+You can suggest that Anymail add an ESP, or even contribute
+your own implementation to Anymail. See :ref:`contributing`.

docs/sending/anymail_additions.rst

@@ -27,6 +27,8 @@ helpful if you are working with Python development tools that
 offer type checking or other static code analysis.
 
 
+.. _anymail-send-options:
+
 ESP send options (AnymailMessage)
 ---------------------------------