Update README.md

Readme: tag example block as js

For great syntax highlighting =D

Readme: Mention possible values for text aligning

Readme: Try making json examples with comments display nicely

Check if 2019's .json5 displays prettily when marked up with 'jsonc'
(json with comments).

Readme: add instructions for writing text to canvas

Readme: Add more info about how to background in scenes

Readme: fix nesting level of heading

Readme: add some more comments

Also, flesh out missing paragraphs, and make description of "how-to-
background" less wall-of-text'y.

Merge pull request #522 from ninjadev/mention-random-in-readme

Mention the existence of Random

Readme: begin describing how to add backgrounds to scenes

Starting with an example of the content of the background node