chore: add dco to contributing

CONTRIBUTING.md

@@ -6,6 +6,8 @@ There are three ways to contribute:
 - **Adding a news highlight** — add a short Markdown file to surface events, releases, or features. [Jump to that section.](#adding-a-news-item)
 - **Changing the site** — UI, CI, or dependencies. Requires a full dev setup. [Jump to that section.](#development-setup)
 
+All contributions must be signed off under the [Developer Certificate of Origin](#developer-certificate-of-origin-dco) — use `git commit -s` on every commit.
+
 ---
 
 ## Adding a blog post
@@ -143,6 +145,75 @@ No config changes or code edits are needed — just the Markdown file.
 
 ---
 
+## Developer Certificate of Origin (DCO)
+
+All contributions to this repository — blog posts, news items, and code changes —
+must be signed off under the [Developer Certificate of Origin](https://developercertificate.org/),
+which certifies that you have the right to submit your work under the project's
+license. By signing off on a commit, you are agreeing to the terms of the DCO
+(full text below).
+
+**Sign off every commit** using `-s` or `--signoff`:
+
+```bash
+git commit -s -m "your commit message"
+```
+
+This appends a `Signed-off-by` trailer using your `user.name` and `user.email` from
+git config:
+
+```text
+Signed-off-by: Jane Doe <jane@example.com>
+```
+
+Use your real name and a reachable email. PRs with unsigned commits will be blocked
+by the DCO check until every commit is signed off. To retroactively sign existing
+commits, use `git rebase --signoff <base>` and force-push.
+
+<details>
+<summary>Developer Certificate of Origin v1.1 (full text)</summary>
+
+```text
+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.
+```
+
+</details>
+
+---
+
 ## Development setup
 
 ```bash