Add getting started / usage to readme

I’ve found that each time I use esquery I have a question how to use it? Is it conventional, like other estools: `var query = require('esquery').query` or is it plain function `var query = require('esquery')`? API is also not clear at first.
Examples could be found in tests, of course, but is there a sense to add short description to the readme, to give new users answer to such questions?
