Update gmail.rst

There are issues with the current docs as they don't address certain user configurations that prevent Symfony from emailing from their app. Mentioning two factor authentication and the less secure app setting is in my opinion, paramount.

Reference: http://stackoverflow.com/q/29085617/1188035

cookbook/email/gmail.rst

@@ -75,3 +75,9 @@ You're done!
 
     The ``gmail`` transport is simply a shortcut that uses the ``smtp`` transport
     and sets ``encryption``, ``auth_mode`` and ``host`` to work with Gmail.
+
+.. note::
+
+    Depending on your Gmail account settings, you may get authentication errors within your app.
+    You should ensure two-factor authentication and [also allow less secure apps to access your
+    account](https://support.google.com/accounts/answer/6010255)