Add support for Helm chart documentation files

[alonsodomin]

Is your feature request related to a problem? Please describe.
As part of the Helm chart file structure there are some documentation-related optional files that a chart may include and which will be helpful for users of a published chart.

The helm_chart target does not support any of those kind of files at the moment, for a more complete implementation, support for these files should be added: values.schema.json, README.md, LICENSE and templates/NOTES.txt.

Describe the solution you'd like
Add one optional single-source field per file to the helm_chart. This would allow to later provide with add-on implementations that could codegen each of those files independently from different sources.

Describe alternatives you've considered
N/A

Additional context
Helm documentation describing chart file structure:
https://helm.sh/docs/topics/charts/