(docs) missing information about throwing errors

[pimlie]

Now that it understand everything it might be logical, but the README doesnt really mention that lock() or the release() returned by lock throws errors. Was looking at the source and it took longer than I want to admit to realize that the callback called in unlock is ofc not visible anywhere when you use async/await syntax (but still used very much so).

A little note about this in the readme would be helpful :)