Add instructions for adding custom components

[haydn]

After the "getting started" section, we'll should explain how to add your own components into config.js.

[Mxchaeltrxn]

@haydn I could probably have a go at this. I don't know if it's worth me writing up the docs (in markdown) and then having you translate it into something visually nice or if you should just write it up and make it look nice in one go.

[haydn]

@Mxchaeltrxn Go for it if you're keen! IMO the most important thing to explain is what each of the fields are when adding a new component into the config (type, options, init, validate and render). Even if there's no visuals at this stage it'll help anyone viewing the website an better understanding of how Sketchbook works.

If you'd like me to review it beforehand, feel free to open a PR.