Cross-reference external script loading from script execution docs

README.md

@@ -84,6 +84,8 @@ dom.window.document.body.children.length === 2;
 
 Again we emphasize to only use this when feeding jsdom code you know is safe. If you use it on arbitrary user-supplied code, or code from the Internet, you are effectively running untrusted Node.js code, and your machine could be compromised.
 
+If you want to execute _external_ scripts, included via `<script src="">`, you'll also need to ensure that they load them. To do this, add the option `resources: "usable"` [as described below](#loading-subresources).
+
 Note that event handler attributes, like `<div onclick="">`, will also not function unless `runScripts` is set to `"dangerously"`. (However, event handler _properties_, like `div.onclick = ...`, will function regardless of `runScripts`.)
 
 If you are simply trying to execute script "from the outside", instead of letting `<script>` elements (and inline event handlers) run "from the inside", you can use the `runScripts: "outside-only"` option, which enables `window.eval`: