website: describe domain in cross provider example

website/source/intro/examples/cross-provider.markdown

@@ -63,6 +63,9 @@ resource "dnsimple_record" "web" {
   ttl = 3600
 }
 
+# The Heroku domain, which will be created and added
+# to the heroku application after we have assigned the domain
+# in DNSimple
 resource "heroku_domain" "foobar" {
     app = "${heroku_app.web.name}"
     hostname = "${dnsimple_record.web.hostname}"