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)

[BenjaminAmos]

Thanks for accepting the game into F-Droid!

I might hold-off merging this for a bit longer, since I can't guarantee that I'll have future builds submitted to F-Droid in a timely fashion. Until the complex source dependency problems are sorted-out, having to write build instructions for new versions of gestalt, joml-ext, nui etc. is a time-consuming task at the moment. These dependencies are evolving with the game and so will almost certainly be updated and upgraded in future releases.