Add a badge/shield to the README as part of the initial PR

What would you like Renovate to be able to do?

As part of the initial Merge/Pull Request that Renovate creates, it’d be nice if the README was updated with a badge/shield as an indicator that Renovate is (going to be) enabled for that repository.

Example shield:

[![Renovate enabled](https://img.shields.io/badge/renovate-enabled-brightgreen.svg?logo=renovatebot)](https://renovatebot.com/)

If you have any ideas on how this should be implemented, please tell us here.

The code for the badge/shield could be added immediately after the header in the README.

Is this a feature you are interested in implementing yourself?

No