Update documentation on AWS response promise

[gfellerph]

The documentation does not mention that if a minified image is stored with AWS the AWS response promise is available with a call to meta() like:

var source = tinify.fromFile("large.jpg");
source.store({
  // ...options
})
  .meta()
  .then((/* meta info? */) => {})
  .catch((err) => {});

I had to read the source to find this. Allthough it was easy, the documentation should mention the possibility.

[tinify-support]

Hi Tuelsch,

Thank you for your input, we will keep this in mind next time we revisit the API documentation.

Happy compressing!

Kind regards,

Ellinor Bergman