use the external-dns slack channel, add section how to become a maint…

[njuettner]

…ainer for provider

Issues:
#357
#358

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[szuecs]

Any provider should have at least one maintainer. It would be nice if you run it in production, but it is not required.
You should check changes and make sure your provider is working correctly.

[ideahitme]

It would be also great to have an automated end-to-end test for different cloud providers, so help from kubernetes maintainers and their idea on how this can be done would be valuable

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[szuecs]

👍

[ideahitme]

awesome 👍