adding `image.flavor` to values.yaml and clarifying `image.tag` in README configuration table

[jessebot]

Pull Request

Description of the change

This just updates the comments for image.tag and image.flavor in values.yaml and README.md to explain where the default tag comes from (appVersion in Chart.yaml) and also that you need to set the flavor to fpm if you'd like to use nginx

Benefits

Gives a bit more clarity on how this chart works, which can be really helpful for new users of nextcloud and/or kubernetes/helm.

Possible drawbacks

none that jump out to me, but happy to hear feedback

Checklist

• [x] DCO has been signed off on the commit.
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] (optional) Variables are documented in the README.md