readme: fix top-level CONTRIBUTING guide

This got missed in the "uber" commit as part of the migration, and still
talked about a CLA. Change that wording to instead reference the DCO,
and defer to the main contributing guide for more details.

Signed-off-by: Paul Jolly <[email protected]>
Change-Id: Iede77d57af0712ab00ea7c25369a5458e0a5881c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/520755

Author: myitcv

CONTRIBUTING.md

@@ -5,25 +5,19 @@ CUE is an open source project.
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
-## Contributor License Agreement
+## Developer Certificate of Origin
 
-Contributions to this project must be accompanied by a Contributor License
-Agreement. You (or your employer) retain the copyright to your contribution;
-this simply gives us permission to use and redistribute your contributions as
-part of the project. Head over to <https://cla.developers.google.com/> to see
-your current agreements on file or to sign a new one.
-
-You generally only need to submit a CLA once, so if you've already submitted one
-(even if it was for a different project), you probably don't need to do it
-again.
+Contributions to the CUE project must be accompanied by a [Developer Certificate
+of Origin](https://developercertificate.org/). Please read the [Contributing
+Guide](./doc/contribute.md) for more details.
 
 ## Code reviews
 
 All submissions, including submissions by project members, require review. We
 use [Gerrit code
 reviews](https://review.gerrithub.io/q/project:cue-lang%252Fcue) for this
 purpose.  Please read the [Contributing Guide](./doc/contribute.md) before
-reading patches.
+submitting patches.
 
 ## Code of Conduct