docs(developer): add domain documentation

Author: Matthew Fisher

docs/developer/config-application.rst

@@ -40,6 +40,35 @@ the application to an external PostgreSQL database.
 
 Detachments can be performed with ``deis config:unset``.
 
+Add Custom Domains to the Application
+-------------------------------------
+
+You can use ``deis domains`` to add or remove custom domains to your application:
+
+.. code-block:: console
+
+    $ deis domains:add hello.bacongobbler.com.com
+    Adding hello.bacongobbler.com to finest-woodshed... done
+
+Once that's done, you can go into your DNS registrar and set up a CNAME from the new
+appname to the old one:
+
+.. code-block:: console
+
+    $ dig hello.deisapp.com
+    [...]
+    ;; ANSWER SECTION:
+    hello.bacongobbler.com.         1759    IN    CNAME    finest-woodshed.deisapp.com.
+    finest-woodshed.deisapp.com.    270     IN    A        172.17.8.100
+
+.. note::
+
+    Setting a CNAME for your root domain can cause issues. Setting your @ record
+    to be a CNAME causes all traffic to go to the other domain, including mail and the SOA
+    ("start-of-authority") records. It is highly recommended that you bind a subdomain to
+    an application, however you can work around this by pointing the @ record to the
+    address of the load balancer (if any).
+
 Track Changes
 -------------
 Each time a build or config change is made to your application, a new :ref:`release` is created.