Update CONTRIBUTING.md: CLA -> DCO (#298)

See connectrpc/connect-query-es#386

Signed-off-by: Dan Rice <[email protected]>

.github/CONTRIBUTING.md

@@ -9,9 +9,8 @@ pull request review much smoother. In your issue, pull request, and any other
 communications, please remember to treat your fellow contributors with
 respect!
 
-Note that you'll need to sign [Buf's Contributor License Agreement][cla]
-before we can accept any of your contributions. If necessary, a bot will remind
-you to accept the CLA when you open your pull request.
+Note that for a contribution to be accepted, you must sign off on all commits
+in order to affirm that they comply with the [Developer Certificate of Origin][dco].
 
 ## Setup
 
@@ -60,5 +59,5 @@ We're much more likely to approve your changes if you:
 
 [fork]: https://github.com/connectrpc/connect-kotlin/fork
 [open-issue]: https://github.com/connectrpc/connect-kotlin/issues/new
-[cla]: https://cla-assistant.io/connectrpc/connect-kotlin
+[dco]: https://developercertificate.org
 [commit-message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html