Missing docs for standard label app.kubernetes.io/created-by

[lindhe]

Currently, all recommended labels except app.kubernetes.io/created-by are mentioned in the docs. I guess we should add it as optional (OPT). What would be a reasonable description and/or example value? I have a hard time coming up with a reasonable default. AFAIK, Helm don't know much about who's installing the release...

https://github.com/helm/helm-www/blob/613bb3f4944ee2c86559e7e74d373a9ce034df1b/content/en/docs/chart_best_practices/labels.md#L35-L43