Reference common text, font, fill, stroke in Readme

[nvkelso]

We top-level list common 3d things like camera and lights in the top-level doc README. We should also list (as a parenthetical or – after the topic headings) text, font, fill, and stroke since those are the most common styling directives.

[meetar]

I think the whole front page probably needs a rewrite. It was originally structured to mirror the structure of the scene file, where the camera and lights are top-level elements, whereas the text options are buried down in the layers block, under draw blocks. The idea was to make it easier to understand how to write a scene file from scratch – but I don't think that's the most common case anymore.

So maybe a separate section of commonly referenced items would be useful – or the doc could be rewritten to prioritize such a list, with a link to a separate "scene file structure reference" document.