Add npm badge

Author: bhch

README.md

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 React Component for editing JSON data using form inputs.
 
+[![npm badge](https://img.shields.io/npm/v/@bhch/react-json-form?color=brightgreen&logo=npm&style=flat-square)][npm]
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 ### Live demo
 
 https://bhch.github.io/react-json-form/
 
 ### Documentation
 
 https://bhch.github.io/react-json-form/
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+[npm]: https://www.npmjs.com/package/@bhch/react-json-form/