message: improve unregister documentation
diff: @@ -2,8 +2,14 @@ export default `import React from "react"
  import useForm from "react-hook-form"
  export default function App() {
  - const { register, handleSubmit, unregister } = useForm()
  - const onSubmit = (data) => console.log(data)
  + const { register, handleSubmit, unregister } = useForm();
  + const onSubmit = (data) => console.log(data);
  +
  + useEffect(() => {
  + register({ name: 'customRegister' }, { required: true });
  +
  + return () => unregister('customRegister'); // unregister input after component unmount
  + }, [register])
  return (
  <form onSubmit={handleSubmit(onSubmit)}>
  @@ -12,5 +18,5 @@ export default function App() {
  <button type="button" onClick={() => unregister('lastName')}>unregister</button>
  <input type="submit" />
  </form>
  - )
  + );
  }`
  