📝 (README.md): add promotional image, update Discord join link, and create table of contents

🌐 (images): add ebook cover image and promotional image

Author: danieldanielecki

README.md

@@ -1,5 +1,7 @@
 # Awesome HTML Book: HyperText Markup Language (HTML)
 
+![Promotional image](images/promotional.png)
+
 [![Join our Discord](images/discord.png 'Join our Discord')](https://discord.gg/RFjtXKfJy3)
 
 ## Table of Contents