Update CONTRIBUTING.md: CLA -> DCO

See connectrpc/connect-query-es#386

Signed-off-by: Dan Rice <drice@buf.build>

.github/CONTRIBUTING.md

@@ -8,9 +8,8 @@ your proposal — discussing API changes ahead of time makes 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
-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
 
@@ -73,3 +72,4 @@ We're much more likely to approve your changes if you:
 [open-issue]: https://github.com/connectrpc/conformance/issues/new
 [commit-message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
 [docker-compose-v2]: https://www.docker.com/blog/announcing-compose-v2-general-availability/#still-using-compose-v1
+[dco]: https://developercertificate.org