🍱 Add css-typed brand image (#20)

Adds css-typed “brand” image to improve README. It combines the CSS 3
and TypeScript logos with a plus. Includes PNG for other needs (example:
the GitHub social media preview) in addition to the primary SVG source.

Author: connorjs

README.md

@@ -1,15 +1,24 @@
+<div align="center">
+
 # css-typed
 
 TypeScript declaration generator for CSS files.
 
-**Table of contents**
+<img alt="css-typed logo" src="./css-typed.svg" width="400">
+
+</div>
+
+<details>
+<summary><strong>Table of Contents</strong></summary>
 
 - [Usage](#usage)
 - [Recipes](#recipes)
 - [Motivation](#motivation)
 - [Contributing](#contributing)
 - [Implementation details](#implementation-details)
 
+</details>
+
 ## Usage
 
 ### Install