Clarify if helm-diff uses `helm upgrade` or `helm template`

[lindhe]

As mentioned in #457, the README for this project seems to initially imply that helm upgrade --debug --dry-run is used for producing the diff. But later, the README states that HELM_DIFF_USE_UPGRADE_DRY_RUN=true must be set in order for it to "use helm upgrade --dry-run instead of helm template to render manifests from the chart".

I find this confusing, so I suggest we try to rephrase the README in order to clarify what's actually the case!