fix: backfill DCO and inbound license grant section in CONTRIBUTING.md (#11)

Tracks DTD#46. Adds the verbatim grant paragraph and Signed-off-by guidance mandated by standards/licensing.md but previously absent from this repo's CONTRIBUTING.md.

Made-with: Cursor

Author: TMHSDigital

CONTRIBUTING.md

@@ -46,3 +46,29 @@ See `.cursorrules` for writing conventions. Key points:
 ## Reporting Issues
 
 Open an issue on GitHub. Include steps to reproduce, expected behavior, and actual behavior.
+
+## Developer Certificate of Origin and Inbound License Grant
+
+This project uses CC-BY-NC-ND-4.0 as its outbound license, which forbids derivatives. Every pull request is a derivative. Contributions are accepted inbound under a broader grant via the Developer Certificate of Origin (DCO), which resolves the conflict so the project can accept and redistribute contributions.
+
+### Required grant
+
+By submitting a contribution to this repository, you certify that you have the right to do so under the Developer Certificate of Origin (DCO) 1.1, and you grant TMHSDigital a perpetual, worldwide, non-exclusive, royalty-free, irrevocable license to use, reproduce, prepare derivative works of, publicly display, publicly perform, sublicense, and distribute your contribution under the project's current license (CC-BY-NC-ND-4.0) or any successor license chosen by the project.
+
+### DCO sign-off
+
+Every commit in a pull request must have a `Signed-off-by:` trailer matching the commit author:
+
+```
+Signed-off-by: Jane Developer <jane@example.com>
+```
+
+Signing is done at commit time:
+
+```bash
+git commit -s -m "feat: add new skill"
+```
+
+The GitHub DCO App enforces this on every PR.
+
+For the full inbound/outbound model and rationale, see [`standards/licensing.md`](https://github.com/TMHSDigital/Developer-Tools-Directory/blob/main/standards/licensing.md) in the Developer-Tools-Directory meta-repo.