Update README.md

[sandercox]

WHen using TypeScript I couldn't get my containerRef to work as it was not of the expected format RefObject<HTMLElement>. I

RefObject expected either HTMLElement or null:

    interface RefObject<T> {
        /**
         * The current value of the ref.
         */
        readonly current: T | null;
    }

Calling useRef without initial arguments creates the ref as MutableRefObject<T | undefined> which is incompatible.

[katspaugh]

Thanks!