📝 docs(readme): Add manual setup and game launch instructions (#69)

## Description

**What issue are you solving (or what feature are you adding) and how
are you doing it?**
Update the README to show how to how to run Netris.

📝 docs: Add README.md (#42)

## Description

**What issue are you solving (or what feature are you adding) and how
are you doing it?**
So, we need a README.md at the root of the project, to at least welcome
anyone who might be interested in the project.

It should have:
- [ ] link to the `/play` endpoint to get a `session_id` for starting up
the server.
 - [ ] (basic) info about how it works
 - [ ] features
 - [ ] images and/or gifs showcasing the project