Update README.md

Author: Alexandre van Beurden

README.md

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 # RegexSolver Node.js API Client
 
-[Homepage](https://regexsolver.com) | [Documentation](https://docs.regexsolver.com) | [Developer Console](https://console.regexsolver.com)
+[Homepage](https://regexsolver.com) | [Online Demo](https://regexsolver.com/demo) | [Documentation](https://docs.regexsolver.com) | [Developer Console](https://console.regexsolver.com)
 
 This repository contains the source code of the Node.js library for [RegexSolver](https://regexsolver.com) API.