docs: add global lib usage instructions (#6294)

See #6033 for context.

Author: filipesilva

docs/documentation/stories/global-scripts.md

@@ -21,3 +21,23 @@ You can also rename the output and lazy load it by using the object format:
   { "input": "pre-rename-script.js", "output": "renamed-script" },
 ],
 ```
+
+## Using global libraries inside your app
+
+Once you import a library via the scripts array, you should **not** import it via a import statement
+in your TypeScript code (e.g. `import * as $ from 'jquery';`).
+If you do that, you'll end up with two different copies of the library: one imported as a
+global library, and one imported as a module.
+
+This is especially bad for libraries with plugins, like JQuery, because each copy will have
+different plugins.
+
+Instead, download typings for your library (`npm install @types/jquery`) which will give you
+access to the global variables exposed by that library.
+
+If the global library you need to use does not have global typings, you can also declare them
+manually in `src/typings.d.ts` as `any`:
+
+```
+declare var libraryName: any;
+```