Clarified how `volume.existingClaim` can be used

[CommanderStorm]

Description of the change

Benefits

Currently the existingClaim option is not usable => with this change it is

Possible drawbacks

I dont know any

Applicable issues

• fixes #71

Additional information

Checklist

• [x] Chart version bumped in Chart.yaml according to semver. ^- I would like to discuss what kind of bump this nessesitates. given this is a bugfix, but also changes the options slightly => minor increment
• [x] Variables are documented in the README.md

[dirsigler]

Hey @CommanderStorm thank you very much for this fix.

Could you please be so kind and bump the version of the Chart here: https://github.com/CommanderStorm/uptime-kuma-helm/blob/volume_existingClaim/charts/uptime-kuma/Chart.yaml#L14 ?

Else the tests don't run properly 😎