plan: Helmfile v1

[mumoshu]

Helmfile v1 may be the first helmfile release may introduce small backward-incompatible changes to provide long-term benefit for users.

It won't break anything badly as Helmfile is already adopted and used widely in production. Don't worry about that.

That said, this is an unordered list of things to be included in Helmfile v1:

• Primarily targeted for use with Helm v3 - Deprecate but not remove v2 support. (Resolved via feat: Experimental Helm v3 mode #668)
• atomic: true by default(atomic: true by default? #512) - Won't break anything in most cases
• helmDefaults renamed to releaseDefaults(Suggestion: renaming "helmDefaults" to "defaults" #521) - helmDefaults will be deprecated but not removed so won't break anything
• Deprecate Double Rendering and recommend new helmfile template instead )(feat: Predictable Helmfile template #932)