[stable/rabbitmq] Add apiVersion in Chart.yaml and add test info to README.md

[carrodher]

According to https://github.com/helm/helm/blob/master/docs/charts.md#the-chartyaml-file:

The Chart.yaml file is required for a chart. It contains the following fields:

apiVersion: The chart API version, always "v1" (required)
name: The name of the chart (required)
version: A SemVer 2 version (required)
...

We're not using the apiVersion field. In the same way, added some information about how we are testing this chart.

[tompizmor]

/ok-to-test
/lgtm

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