add a warning about the difference between Swiftmailer DSNs and Mailer ones

Author: fabpot

mailer.rst

@@ -33,6 +33,11 @@ can deliver emails over ``smtp`` by configuring your ``.env`` file:
     # .env
     MAILER_DSN=smtp://user:pass@smtp.example.com
 
+.. warning::
+
+    If you are migrating from Swiftmailer (and the Swiftmailer bundle), be
+    warned that the DSN format is different.
+
 Using a 3rd Party Transport
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~