Update CONTRIBUTING.md (#7852)

Adds a note for who can merge PR.

CONTRIBUTING.md

@@ -10,6 +10,9 @@ The code for any model in this repository is licensed under the Apache License
 In order to accept our code, we have to make sure that we can publish your code:
 You have to sign a Contributor License Agreement (CLA).
 
+***NOTE***: Only [code owners](./CODEOWNERS) are allowed to merge a pull request. 
+Please contact the code owners of each model to merge your pull request.
+
 ### Contributor License Agreements
 
 Please fill out either the individual or corporate Contributor License Agreement (CLA).