Update md file by adding a simple example

[EZZAHED]

the example is copied form here https://github.com/final-form/react-final-form-hooks

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 557ef29:

Sandbox	Source
React Final Form - Simple Example	Configuration
React Final Form - Synchronous Record Level Validation	Configuration
React Final Form - Synchronous Field Level Validation	Configuration
React Final Form - Submission Errors	Configuration
React Final Form - Subscriptions Example	Configuration

[mrfratello]

This example is not suitable, since the hooks are taken from 'react-final-form-hooks'. It uses another api