Add a comment on pull request when a non collaborator submit a pull request

[chmouel]

When a non collaborator user submit a pull request instead of being completely quiet about it we can add a comment letting know the user waiting for some collaborator to issue a /ok-to-test

an example of how prow message look like :

Hi username. Thanks for your PR.

I'm waiting for a [tektoncd](https://github.com/orgs/tektoncd/people) member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should [join the org](https://github.com/orgs/tektoncd/people) to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed [here](https://go.k8s.io/bot-commands?repo=tektoncd%2Fpipeline).

cc @sm43 :)

[sm43]

would this be additional to the checkrun we create or we skip creating the checkrun?

[chmouel]

I don't think a checkrun will be created since people has been explicitly not wanting that and that's not how prow works too..