docs: Add DCO

Signed-off-by: Slendi <slendi@socopon.com>

CONTRIBUTING.md

@@ -54,6 +54,20 @@ This project uses `clang-format` to maintain a consistent code style. Before ope
 git ls-files '*.hpp' '*.cpp' | xargs clang-format -i
 ```
 
+## Sign-off (DCO)
+
+Every commit must be signed off by at least one (1) human contributor:
+
+```
+git commit -s
+```
+
+This adds a `Signed-off-by: Your Name <your@email>` trailer certifying that you
+wrote the change (or otherwise have reviewed the commit and have the right to
+submit it) and agree to contribute it under the project's license, per the
+Developer Certificate of Origin (the DCO file in this repository - also at
+https://developercertificate.org).
+
 ## AI-assisted contributions
 
 AI-assisted contributions are allowed, subject to the following:

DCO

@@ -0,0 +1,35 @@
+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.
+