Update CONTRIBUTING.md: CLA -> DCO (#1707)

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

.github/CONTRIBUTING.md

@@ -8,9 +8,11 @@ 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].
+Make sure to configure `git` with the same name and E-Mail as your GitHub account,
+and run `git commit` with the `-s` flag to sign. If necessary, a bot will remind
+you to sign your commits when you open your pull request, and provide helpful tips.
 
 ## Setup
 
@@ -91,5 +93,5 @@ After updating dependencies, make sure to run all tests and checks with `./manag
 [react-native-guide]: https://react-native-community.github.io/upgrade-helper/)
 [fork]: https://github.com/connectrpc/examples-es/fork
 [open-issue]: https://github.com/connectrpc/examples-es/issues/new
-[cla]: https://cla-assistant.io/connectrpc/examples-es
+[dco]: https://developercertificate.org
 [commit-message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html