v6.2.0-alpha.1

• Only instantiating a ResizeObserver instance if there's actually something to
  observe. This for example means that if you pass in null or undefined as the
  ref, or if neither the default ref or callback ref returned from the hook are
  in use, then no ResizeObserver instance will get created until there's an
  actual element to observe. Resolves: #42
• The hook now returns callbackRef, which can be used in place of the usual
  ref. Use this instead of a normal ref, when the observed component is
  mounted with a delay. Resolves: #43, #45
• The ref option now accepts raw elements as well.
• Handling custom refs (through options), the default ref and the callback ref
  has been greatly refactored internally (into the useResolvedElement
  hook), to handle more edge cases with the way refs are handled.
• Tests based on react testing library were refactored to make them much simpler
  and more approachable.
• Fixed an error where in certain edge cases the hook tried to set state when
  its host component already unmounted.
• Added contributing guidelines
• Overall bundle size increased a bit, due to the new features added.
  (With about ~150B or so.)

As this is an alpha release, feedback are welcome.