add repository mirror and issue tracking notice
To prevent fragmentation of feedback, we want all bug reports and
contributions to be centralized on our main self-hosted platform.
GitHub lacks active monitoring for issues, so users need clear
direction on where to officially open them.
Adds a proper project README (description, tech stack, structure,
setup, scripts) and a CONTRIBUTING guide (governance summary,
workflow, PR expectations), both previously missing or placeholder-only.
Ninosaurier