(#2923) - add blob-util to docs

I've been working with Blobs and image data a lot at work recently, and it's really hard. Especially when it comes to supporting "older" browsers (and by "older," I mean Android \<4.4 of course \>_\<).

So I wrote [a blob library](https://github.com/nolanlawson/blob-util) to make stuff easier. This commit is a shameless plug for that library.