[Doc] Add the DCO to CONTRIBUTING.md (#9803)

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Co-authored-by: Michael Goin <michael@neuralmagic.com>
Co-authored-by: Cyrus Leung <cyrus.tl.leung@gmail.com>

CONTRIBUTING.md

@@ -11,12 +11,14 @@ We also believe in the power of community support; thus, answering queries, offe
 
 Finally, one of the most impactful ways to support us is by raising awareness about vLLM. Talk about it in your blog posts and highlight how it's driving your incredible projects. Express your support on social media if you're using vLLM, or simply offer your appreciation by starring our repository!
 
+## License
+
+See [LICENSE](LICENSE).
 
 ## Developing
 
 Depending on the kind of development you'd like to do (e.g. Python, CUDA), you can choose to build vLLM with or without compilation. Check out the [building from source](https://docs.vllm.ai/en/latest/getting_started/installation.html#build-from-source) documentation for details.
 
-
 ## Testing
 
 ```bash
@@ -33,6 +35,14 @@ pytest tests/
 
 ## Contribution Guidelines
 
+### DCO and Signed-off-by
+
+When contributing changes to this project, you must agree to the [DCO](DCO).
+Commits must include a `Signed-off-by:` header which certifies agreement with
+the terms of the [DCO](DCO).
+
+Using `-s` with `git commit` will automatically add this header.
+
 ### Issues
 
 If you encounter a bug or have a feature request, please [search existing issues](https://github.com/vllm-project/vllm/issues?q=is%3Aissue) first to see if it has already been reported. If not, please [file a new issue](https://github.com/vllm-project/vllm/issues/new/choose), providing as much relevant information as possible.

DCO

+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.