Update README.md

Author: bluebill1049

README.md

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+<div align="center"><h1>📣 UPDATE</h1>
+
+This component is now a part of React Hook Form V4, and renamed to <a href="https://react-hook-form.com/api/#ErrorMessage">ErrorMessage</a>.
+
+</div>
+
 <div align="center">
     <p align="center">
         <a href="https://react-hook-form.com" title="React Hook Form - Simple React forms validation">