Add 2-cpp approvers text to contributing guide [no ci] (#17182)

Adds text to the contributing guide mentioning 2 cpp-codeowner approvals are required for any C++changes.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Nghia Truong (https://github.com/ttnghia)

URL: #17182

CONTRIBUTING.md

@@ -38,6 +38,7 @@ conduct. More information can be found at:
 8. Verify that CI passes all [status checks](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/about-status-checks).
    Fix if needed.
 9. Wait for other developers to review your code and update code as needed.
+   Changes to any C++ files require at least 2 approvals from the cudf-cpp-codeowners before merging.
 10. Once reviewed and approved, a RAPIDS developer will merge your pull request.
 
 If you are unsure about anything, don't hesitate to comment on issues and ask for clarification!