Add APIGuard ad strip above Table of Contents

Author: mkhorasani

README.md

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 <!--- ###### _To learn more please refer to my book [Web Application Development with Streamlit](http://tinyurl.com/2p8mw32d)._ --->
 
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+> <a href="https://apiguard.co"><img src="https://apiguard.co/favicon.svg" height="18" align="center" /></a> &nbsp; **[APIGuard](https://apiguard.co)** — Automatically test your API against its OpenAPI spec on every deploy. Catch breaking changes, schema drift, and contract violations before they reach production. **[Try it free →](https://apiguard.co/register)**
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 ## Table of Contents
 - [Quickstart](#1-quickstart)
 - [Installation](#2-installation)