docs(project): add Citation section with BibTeX to README

Adds an explicit Citation section before Governance so researchers can
copy a canonical BibTeX entry without clicking through to Zenodo. Uses
the same author list and concept DOI as CITATION.cff so the citation
always resolves to the latest release. Follows the pattern used by
borglab/gtsam and huggingface/transformers.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

README.md

@@ -104,6 +104,22 @@ make px4_sitl
 
 We welcome contributions of all kinds — bug reports, documentation, new features, and code reviews. Please read the [Contribution Guide](https://docs.px4.io/main/en/contribute/) to get started.
 
+## Citation
+
+If you use PX4 in academic work, please cite it. BibTeX:
+
+```bibtex
+@software{px4_autopilot,
+  author    = {Meier, Lorenz and {The PX4 Contributors}},
+  title     = {{PX4 Autopilot}},
+  publisher = {Zenodo},
+  doi       = {10.5281/zenodo.595432},
+  url       = {https://px4.io}
+}
+```
+
+The DOI above is a Zenodo concept DOI that always resolves to the latest release. For a version-pinned citation, see the [Zenodo record](https://doi.org/10.5281/zenodo.595432) or our [`CITATION.cff`](CITATION.cff).
+
 ## Governance
 
 The PX4 Autopilot project is hosted by the [Dronecode Foundation](https://www.dronecode.org/), a [Linux Foundation](https://www.linuxfoundation.org/) Collaborative Project. Dronecode holds all PX4 trademarks and serves as the project's legal guardian, ensuring vendor-neutral stewardship — no single company owns the name or controls the roadmap. The source code is licensed under the [BSD 3-Clause](LICENSE) license, so you are free to use, modify, and distribute it in your own projects.