Please fix the README with some documentation

[keul]

Sorry but the current readme file required me to spend 20 minutes trying to figure out what's going on and read the source code.

For example: there's no reference to the .observe or .unobserve methods (and the first ones seems required to make this working).

A new readme proposal (taken from https://codesandbox.io/s/ko5wqqz617)

let rectObserver = observeRect(node, rect => {
  console.log(rect.left, rect.top, rect.height, rect.width);
});
rectObserver.observe();

(also a link to a codesandbox could help)