Add core docs: README, CONTRIBUTING, Roadmap

### Description

The repo is missing basic documentation. We need a minimal setup so people can understand the project and contribute without friction.

### Tasks

* **README.md**

* what the project does / why it exists
* quickstart (install + example)
* link to roadmap + contributing
* clarify relation to upstream (since this is a fork)

* **CONTRIBUTING.md**

* how to run the project locally
* branch naming (`feat/`, `fix/`, etc.)
* PR expectations (small, clear, tests if relevant)
* commit style (`feat:`, `fix:`, `docs:`…)

* **Roadmap**

* simple structure: Now / Next / Later
* main priorities (stability, CI, tests, core features)

### Done when

* README is enough to get started quickly
* CONTRIBUTING is usable without asking questions
* roadmap is visible and clear
* links between docs are consistent