Update V3.x ReadMe to Bring Back "Declarative" Message

[seth-acuitymd]

TL;DR

The v2 README has a block of text near the top that states

This GitHub Action is declarative, meaning it will overwrite any values on an existing Cloud Function deployment. If you manually deployed a Cloud Function, you must specify all parameters in this action. Any unspecified values will be reverted to their default value (which is usually "null").

The V3 README does not have that text, as such - when using the v3 action, I accidentally defaulted a bunch of config I was specifying via Terraform (that's why we test in lower envs first 😄 )

Detailed design

No response

Additional information

If this GitHub Action is not meant to be declarative in the way that v2 was, then there's a bug, but I assume since v2 is declarative, that v3 will be as well