Document support for using with react-dropzone

I've personally integrated this with react-dropzone, but the code was too specific to my use case to release.

We should add some code and documentation for the preferred way to integrate with react-dropzone.

For those wondering, I essentially bypassed the ReactS3Uploader react component and just included s3upload.js directly. Using the S3Upload class, I just wrapped each dropped file in a new instance, which kicks off the same upload process as using the component.