docs: comprehensive README documenting all fork differences from upstream ppy/osu

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/1d3c6971-e9fd-4074-9f8e-0bcb8c85965d

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot