Update README.md with F-Droid link

[Poussinou]

Description

This PR adds a link in the readme to get the app on F-Droid

Testing

Outstanding Work

• [ ] If there is work still outstanding, please include it here
• [ ] if there is nothing outstanding, remove this section.

Notes

Thanks for creating this game and including it in F-Droid :)

Pre Pull Request Checklist:

• [ ] Code has been scanned with SonarLint
• [ ] There are no errors present in the project
• [ ] Code has been formatted and indented
• [ ] Methods have appropriate Javadoc (How to write Javadoc)