doc: add a quick guide to the library

This is a nice intro to the documentation, and also preferably gently
pushes users to the "proper way" of using the library, which can be
confusing.

See also: http://fooishbar.org/tell-me-about/xkbcommon-intro/

Signed-off-by: Ran Benita <ran234@gmail.com>
Reformat README markdown

So that github displays it as markdown, and correctly.

Signed-off-by: Ran Benita <ran234@gmail.com>