Add CONTRIBUTING

Signed-off-by: Pol Henarejos <pol.henarejos@cttc.es>

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to this project.
+
+This repository is published in two forms:
+- a Community Edition released under AGPLv3, and
+- a proprietary / commercial / Enterprise Edition offered to organizations.
+
+To keep that model legally clean, we need to be explicit about how contributions can be used.
+
+By opening a pull request, you agree to all of the following:
+
+1. **You have the right to contribute this code.**
+   You are either the original author of the contribution, or you have obtained the necessary rights/permissions to contribute it under these terms.
+
+2. **Dual licensing permission.**
+   You agree that your contribution may be:
+   - merged into this repository, and
+   - used, copied, modified, sublicensed, and redistributed
+     - under the AGPLv3 Community Edition, and
+     - under any proprietary / commercial / Enterprise editions of this project,
+       now or in the future.
+
+   In other words: you are granting the project maintainer(s) the right to include
+   your contribution in both the open-source (AGPLv3) codebase and in closed-source /
+   commercially licensed builds, without any additional approval or payment.
+
+3. **Attribution.**
+   The maintainers may keep or add attribution lines such as
+   `Copyright (c) <your name>` or an AUTHORS / CONTRIBUTORS list.
+   The maintainers may also make changes for clarity, style, security, refactoring,
+   or integration reasons.
+
+4. **No automatic SLA.**
+   Submitting a pull request does *not* create any support obligation,
+   service-level agreement, warranty, or guarantee that the contribution
+   will be reviewed, merged, or maintained.
+
+5. **Potential rejection for business reasons.**
+   Features that fall under "Enterprise / Commercial" functionality
+   (e.g. multi-tenant provisioning at scale, centralized audit trails,
+   corporate policy enforcement, attestation/branding flows, key escrow / dual-control,
+   etc.) may be declined for the public AGPLv3 tree even if technically valid.
+   That is normal: some functionality is intentionally offered only
+   under commercial terms.
+
+If you are not comfortable with these terms, **do not open a pull request yet.**
+Instead, please open an Issue to start a discussion.
+
+## How to contribute (technical side)
+
+### 1. Bug reports / issues
+- Please include:
+  - hardware / board revision
+  - firmware / commit hash
+  - exact steps to reproduce
+  - expected vs actual behavior
+  - logs / traces if available (strip secrets)
+
+Security-sensitive findings: do **not** post publicly.
+Send a short report by email instead so it can be triaged responsibly.
+
+### 2. Small fixes / minor improvements
+- You can open a PR directly for:
+  - bug fixes
+  - portability fixes / new board definitions
+  - clarifications in code comments
+  - build / tooling cleanup
+  - documentation of existing behavior
+
+Please keep PRs focused (one logical change per PR if possible).
+
+### 3. Larger features / behavior changes
+- Please open an Issue first and describe:
+  - what problem you're solving (not just "add feature X")
+  - impact on existing flows / security model
+  - any new dependencies
+
+This helps avoid doing a bunch of work on something that won't be accepted
+in the Community Edition.
+
+### 4. Coding style / security posture
+- Aim for clarity and small, auditable changes. This code runs in places
+  where secrets live.
+- No debug backdoors, no "just for testing" shortcuts left enabled.
+- Keep external dependencies minimal and license-compatible
+  (MIT / Apache 2.0 / similarly permissive is usually fine).
+
+### 5. Commit / PR format
+- Use descriptive commit messages ("Fix PIN retry counter wrap" is better than "fix stuff").
+- In the PR description, please include a short summary of what was changed and why.
+- At the bottom of the PR description, **copy/paste and confirm the licensing line below**:
+
+  > I confirm that I have read `CONTRIBUTING.md` and I agree that this contribution may be used under both the AGPLv3 Community Edition and any proprietary / commercial / Enterprise editions of this project, now or in the future.
+
+A PR without that confirmation may be delayed or closed without merge.
+
+## Thank you
+
+This project exists because people build on it, break it, fix it,
+and push it into places it wasn't originally designed to go.
+
+Whether you are here for research, hacking on hardware,
+rolling out secure keys for a team, or building a commercial product:
+thank you for helping improve it.