Support library charts

[Anticom]

Following this guide for library charts there is no value file and no templates that produce yaml directly but only helpers.
It would be nice if helm-docs wasn't skipping the README generation upon missing values file but still render everything from the Chart.yaml meta-data so we get this part documented automatically.

This could either be implemented using a --library-chart etc. flag or as a breaking change where it doesn't skip the generation when the values file is missing.

[mccurdyc]

@norwoodj Love what helm-docs provides, thank you! if someone were to implement this suport for Helm Library Charts, how would you suggest they do it?

The Helm Library Chart docs explicitly recommend deleting the values.yaml file.

But for helm-docs purposes, we could keep the values.yaml file which would just contain comments and just use the old helm-docs comment format where you explicitly specify field names and accompanying comment.

But, is this really any better than supporting a values.schema.json file? #90

[skang0601]

Closing in favor of #90, having some spare cycles to start working on support that work.