Helm Chart: Add NOTES.txt template & update chart.yaml

[richard-cox]

• Add NOTES template
• Add maintainers and keywords to chart.yaml
• contributes to Add Stratos helm chart repo to Helm Hub cloudfoundry/stratos#4556

helm install looks like...

richard@linux-o35c ~/dev/github/stratos-metrics/build (notes-and-maintainers) $ helm status my-metrics -n my-metrics
NAME: my-metrics
LAST DEPLOYED: Thu Sep  3 14:16:47 2020
NAMESPACE: my-metrics
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
Thank you for installing Stratos Metrics. Your release is named 'my-metrics' and is in the namespace 'my-metrics'.

To learn more about the release, try:
  $ helm status my-metrics -n my-metrics
  $ helm get values my-metrics -n my-metrics
  $ kubectl get services -n my-metrics
  $ kubectl get pods -n my-metrics

[nwmac]

LGTM