Add documentation about big numbers #42

I could have sworn I had written something in the readme about it, but apparently I didn't.

For reference for anyone finding this before I actually get around to adding it to the docs:

We ran into an issue #16 which is that number values returned from Datadog were larger than JSON could support, so there was lose of precision/overflowing for large numbers.

The solution was to utilize json-bigint which utilizes bignumber.js to support larger numbers in JSON.

The problem with this is that the format that bignumber.js stores numbers in isn't the most intuitive manner.

> var bn = require('bignumber.js');
> console.dir(new bn('2349082309420934834234234'));
{ s: 1, e: 24, c: [ 23490823094, 20934834234234 ] }

For information on how to interact with bignumber.js please see their documentation: https://www.npmjs.com/package/bignumber.js