Clarify the pull request review guidance for company diversity

As discussed at the governance/community meeting on 25/10, we clarify
the guidance on company diversity for reviews. This update upholds that
company diversity is an important recommendation but that maintainers
have discretion to make exceptions after careful review of the changes.

process/README.md

@@ -129,9 +129,10 @@ The configuration for this automation is in
 _More on the Prow process in general
 [is available in the k8s docs](https://github.com/kubernetes/community/blob/master/contributors/guide/owners.md#the-code-review-process)._
 
-The Tekton community promotes company diversity as a best practice for pull request.
+The Tekton community promotes company diversity as a best practice for pull requests.
 This means that, where possible, one of the reviewers of a pull request and the author
-should be affiliated to different organizations.
+should be affiliated to different organizations. Exceptions to this guidance may be made
+at the discretion of maintainers after careful review of the changes.
 
 This best practice may not be applicable by all Tekton projects, please check the
 guidelines on a project specific details for more details.