Mailgun: Document how to use European region

Closes #116

CHANGELOG.rst

@@ -43,6 +43,8 @@ Breaking changes
 Features
 ~~~~~~~~
 
+* **Mailgun:** Document how to use new European region. (This works in earlier
+  Anymail versions, too.)
 * **Postmark:** Add support for Anymail's normalized `metadata` in sending
   and webhooks.
 

docs/esps/mailgun.rst

@@ -62,8 +62,17 @@ The base url for calling the Mailgun API. It does not include
 the sender domain. (Anymail :ref:`figures this out <mailgun-sender-domain>`
 for you.)
 
-The default is ``MAILGUN_API_URL = "https://api.mailgun.net/v3"``
-(It's unlikely you would need to change this.)
+The default is ``MAILGUN_API_URL = "https://api.mailgun.net/v3"``, which connects
+to Mailgun's US service. You must override this if you are using Mailgun's European
+region:
+
+  .. code-block:: python
+
+      ANYMAIL = {
+        "MAILGUN_API_KEY": "...",
+        "MAILGUN_API_URL": "https://api.eu.mailgun.net/v3",
+        # ...
+      }
 
 
 .. _mailgun-sender-domain: