Add unified CONTRIBUTING.md draft (Issue #186)

[vaibhav11123]

Fixes #186

This PR adds a first draft of a unified CONTRIBUTING.md.

The goal is to provide a simple and friendly starting point that reflects the common contribution practices already used across AboutCode projects. This version is intentionally lightweight — no new policies or tooling — just a clearer, more consistent guide for contributors.

Happy to update the wording, structure, or placement based on feedback from maintainers. Thanks!