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!

[yxxhero]

Set HELM_DIFF_USE_UPGRADE_DRY_RUN=true to

use helm upgrade --dry-run instead of helm template to render manifests from the chart.

See #253 for more information.

HELM_DIFF_USE_UPGRADE_DRY_RUN=true helm diff upgarde my-release datadog/datadog