Add: Redis persistent volumes settings to values

[f3k-freek]

Pull Request

Description of the change

Added Redis persistent volume settings to values and the readme. This is only to make users aware of how to change these settings, the PR itself doesn't contain any changes to the functioning of the chart.

Benefits

It will make people aware of how to change the persistent volume settings for Redis.

Possible drawbacks

Too many settings in values might "clutter" the values file and readme.

Applicable issues

• fixes #546

Additional information

not applicable

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