Write documentation

[davcri]

• [x] Update Readme
• [x] How to change scene (multithread, single thread, ...)
• [x] How to change scene and pass parameters
• [x] How to play a specific scene
• [x] How to restart the current scene (with no params and with latest params)
• [x] FAQ/common errors: get_tree().get_node().add_child(new_scene)
• [x] Godot API Incompatibilities (reload_current_scene(), goto_scene(), ...)
• [x] How to disable CI and why
• [ ] How to contribute (main branch, feature branch, PR, changelog)
• [ ] How to play sounds across scene changes
• [ ] How to play BGM across scene changes
• [ ] How to persiste game data (across scene changes)
• [ ] How to persist game data (across application restarts)