README: Add showcase: missionbit.org

missionbit.org uses next-on-netlify, so let's feature it in the README!

For details, see: 
#18 (comment)

README.md

@@ -7,11 +7,25 @@
 
 next-on-netlify is a utility for hosting NextJS applications with Server-Side Rendering on Netlify. It wraps your NextJS application in a tiny compatibility layer, so that pages can be server-side rendered with Netlify functions.
 
-
-
 - Demo: https://next-on.netlify.com/  
 - Example repository: https://github.com/FinnWoelm/next-on-netlify-demo
 
+## Table of Contents
+* [Installation](#installation)
+* [Setup](#setup)
+  - [1. Set NextJS  target to serverless](#1-set-nextjs--target-to-serverless)
+  - [2. Add postbuild hook](#2-add-postbuild-hook)
+  - [3. Configure Netlify](#3-configure-netlify)
+* [Optional Extras](#optional-extras)
+  - [Preview Locally](#preview-locally)
+  - [Custom Netlify Redirects](#custom-netlify-redirects)
+  - [Custom Netlify Functions](#custom-netlify-functions)
+* [Caveats](#caveats)
+  + [Preview Mode](#preview-mode)
+  + [Fallbacks for Pages with `getStaticPaths`](#fallbacks-for-pages-with--getstaticpaths-)
+* [Showcase](#showcase)
+* [Credits](#credits)
+
 ## Installation
 
 ```
@@ -147,6 +161,15 @@ With `next-on-netlify`, when navigating to a path that is not defined in `getSta
 
 For more on this, see: [Issue #7](https://github.com/FinnWoelm/next-on-netlify/issues/7#issuecomment-636883539)
 
+## Showcase
+
+The following sites are built with `next-on-netlify`:
+
+![missionbit.org](https://raw.githubusercontent.com/FinnWoelm/next-on-netlify/assets/showcase-missionbit.png)  
+[missionbit.org](https://www.missionbit.org/) ([#18](https://github.com/FinnWoelm/next-on-netlify/pull/18#issuecomment-643828966))
+
+Are building something awesome with `next-on-netlify`? 🔥 Let me know and we will feature it here :)
+
 ## Credits
 
 📣 Shoutout to [@mottox2](https://github.com/mottox2) (a pioneer of hosting NextJS on Netlify) and [@danielcondemarin](https://github.com/danielcondemarin) (author of serverless-next.js for AWS). The two were big inspirations for this package.