[Encore] FAQ / common problems section

[weaverryan]

I think a FAQ or "common problems" section would be a big help. This would initially include:

• Uncaught ReferenceError: webpackJsonp is not defined (see webpackJsonp is not defined webpack-encore#42)
• jQuery is not defined or $ is not defined (it depends on where this error happens - either the user needs to expose these as a global variable [e.g. if this error happens in JS code written in a template] or they may need autoProvidejQuery() if this happens in a required module.