[stable/mariadb] Document the correct format for pullSecrets in README

[carrodher]

Signed-off-by: Carlos Rodriguez Hernandez [email protected]

What this PR does / why we need it:

XXX.pullSecrets should be an array of secrets, this PR add some comments in the README to clarify it.
Basically, it replaces different ways of

| `image.pullSecrets` | Specify image pull secrets | `nil` |

by

| `image.pullSecrets` | Specify docker-registry secret names as an array | `[]` (does not add image pull secrets to deployed pods) |

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md

[juan131]

/lgtm

[rimusz]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: carrodher, juan131, rimusz

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[k8s-ci-robot]

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