Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

[prometheus addon] Add readme

This PR adds README file that warns users about it's intended use, and redirects to more mature projects.

```release-note
NONE
```
/cc @brancz