Document various KISS-UI features

[abonander]

• Basics: initialize KISS-UI, create a dialog with a couple widgets
• Using callbacks with and without associated data; implementing Callback for user structs
• Extending KISS-UI using hidden APIs

Addendum:

• Move installation instructions out of README and into new topic

Suggestions for more topics are welcome.

RFC: use the Github wiki system or create a new Markdown file for each topic? Looking for discussion in this reddit thread.