docs: clarify that helm-diff uses helm template by default (#960)

The README intro incorrectly implied that helm-diff uses
helm upgrade --debug --dry-run by default. In reality, it uses
helm template by default, and helm upgrade --dry-run is only used
when HELM_DIFF_USE_UPGRADE_DRY_RUN=true is set.

Fixes #666

Signed-off-by: yxxhero <aiopsclub@163.com>

Author: yxxhero

README.md

@@ -6,8 +6,8 @@
 
 This is a Helm plugin giving you a preview of what a `helm upgrade` would change.
 It basically generates a diff between the latest deployed version of a release
-and a `helm upgrade --debug --dry-run`. This can also be used to compare two
-revisions/versions of your helm release.
+and a `helm template`-rendered manifest (or `helm upgrade --dry-run` when `HELM_DIFF_USE_UPGRADE_DRY_RUN=true` is set).
+This can also be used to compare two revisions/versions of your helm release.
 
 <a href="https://asciinema.org/a/105326" target="_blank"><img src="https://asciinema.org/a/105326.png" /></a>