docs(introduction): clarify usage of functions in config.js for browser availability

[KristjanESPERANTO]

"Solves" MagicMirrorOrg/MagicMirror#4146.

[sdetweil]

as you are updating configuration, could you add this

/*************** DO NOT EDIT THE LINE BELOW ***************/
if (typeof module !== "undefined") {module.exports = config;}

to the end of the example of minimum config, item 3

we've had a couple users do this, instead of copying the sample and it fails (at least now there is a message)

[KristjanESPERANTO]

Done 🙂