async script support or note

I'm not sure how much of an issue it is, but when the JS is loaded as <script async src="path/to.js"> it doesn't execute. It doesn't appear to note any issues in the console or raise any exceptions, but it doesn't do anything. The spinner appears, the script shows in the sources panel, but nothing actually happens.

If I switch to <script defer src="path/to.js"> so that the script executes as soon as the DOM is loaded (from my very limited knowledge!) then everything seems to be ok.

Perhaps a small note in the readme about this, a change to make the script work with async if that's relatively trivial, would help people not run across the same issue I had.