docs: add link to Code of Conduct to CONTRIBUTING guide

The CONTRIBUTING guide is also now mirrored on cuelang.org website at
/docs/contribution_guidelines/

Change-Id: I7079775e32faa750c92d9f3737adf9c7f1f017d4
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9541
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

CONTRIBUTING.md

@@ -24,7 +24,8 @@ use Gerrit code reviews at https://cue-review.googlesource.com for this purpose.
 Please read the [Contributing Guide](./doc/contribute.md)
 before reading patches.
 
-## Community Guidelines
+## Code of Conduct
 
-This project follows [Google's Open Source Community
-Guidelines](https://opensource.google.com/conduct/).
+Guidelines for participating in CUE community spaces and a reporting process for
+handling issues can be found in the [Code of
+Conduct](https://cuelang.org/docs/contribution_guidelines/conduct).