Add getting started / usage to readme

[dy]

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?

[kristianmandrup]

Please add proper docs. Someone please make a PR with good examples and overview of how to use the API. Cheers!

[mandricore]

I made this PR with usage docs