Update readme with renovate configuration for digest updates (#514)

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### Description of the change

Update README with information how to configure renovate to update digest for gitea charts.

### Benefits

Automatic digest updates for people using renovate.

Co-authored-by: Michał Małyska <999598+mmalyska@users.noreply.github.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/514
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Reviewed-by: pat-s <pat-s@noreply.gitea.com>
Co-authored-by: mmalyska <mmalyska@noreply.gitea.com>
Co-committed-by: mmalyska <mmalyska@noreply.gitea.com>

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Gitea Version: 1.28.0+dev-528-ga5a645824f