Add information on how to debug an extension

[luceos]

While talking to @tobscure he offered some advice on debugging extensions which use the front-end (mithrill) side:

• check whether your extension is loaded System.get('vendor/extension/main'); in the source of the page
• inspect the app
• in the console: System.get('path/to/module').default to inspect a module/class

Might need more than this. But this is a start.

[luceos]

Related: https://discuss.flarum.org/d/1662-extension-developer-first-read

[KyrneDev]

Woah, this is an old issue. I think this is important info but should be brought into the 20s