Add page explaining how to send template using API

Would like to test something like this and see how well it works.

Intention of having this page is so:
- template IDs are discoverable (https://www.pivotaltracker.com/story/show/115404593)
- it’s obvious there’s an ‘automated’ way to send messages, as well as the CSV
  way (we’ve seen people oblivious to this in research)

app/assets/stylesheets/app.scss

@@ -65,6 +65,7 @@ a {
 .highlight {
   font-family: monospace;
   overflow-x: scroll;
+  padding: 10px 0 10px 10px;
 }
 
 .inline {