Update CONTRIBUTING.md - add testing tips and screenshot for allow edits by maintainers

[jessebot]

Pull Request

Description of the change

This add some more items to the contributing docs to check before submitting a PR, and it now briefly goes over testing the changes locally.

Benefits

Users can learn about helm lint, helm template, and the ct (chart testing) command. These are all tools by the helm project itself.

Possible drawbacks

more to read :P

Applicable issues

n/a

Additional information

n/a

Checklist

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

[jessebot]

We should probably also add how to make a PR editable by maintainers, so that we can bump helm chart versions and resolve (minor) conflicts if necessary. :thinking:

[jessebot]

I've updated the docs to include a screenshot of how to allow edits by maintainers. @provokateurin or @tvories could you please review? Thank you :pray:

[jessebot]

thank you!