Add Introduction to Readme

[oleima]

Is your feature request related to a problem? Please describe.
There should exist a Introduction section in the Readme. Using this markdown guide to create the section, it can look nice too.

Describe the solution you'd like
A heading level 2 with the text Introduction and then a short description in normal text shortly describing that this is a repo aiming to write a book hosted in GitHub Pages.

Describe alternatives you've considered
Use the guide provided, ask for help from the workshop presenters, or any source to learn basic markdown.

Additional context
This change must be done in a seperate branch, and merged via pull requests. Acceptance criteria is that there is correct markdown heading 2 containing the word Introduction and that there is a simple explanation in normal text below. Do this with a friend if you are unsure, or attempt to solve it by yourself.