add injector/gtm instructions

Some consumers of the CDN may be using Drupal asset injector or js injector which supports adding arbitrary inline js to a page but not external js. We should add some instructions for how to add the chatbot in these scenarios.

Drupal 7 / JS Injector

var scriptTag = document.createElement('SCRIPT');
scriptTag.src = '//assets.pinterest.com/js/pinit.js';
scriptTag.async = true;
scriptTag.defer = true;
document.body.appendChild(scriptTag);

Drupal 8 / Asset Injector

$.getScript("//domain.com/path/to/file.js");