Improve local dev override docs and add read-only example

examples/read_only/providers.tf

@@ -0,0 +1,18 @@
+terraform {
+  required_version = ">= 1.5.0"
+
+  required_providers {
+    dynu = {
+      source = "dynu/dynu"
+    }
+  }
+}
+
+# Local development note:
+# This source address stays "dynu/dynu" while using ~/.terraformrc dev_overrides.
+# Terraform will load your local terraform-provider-dynu binary instead of the public registry.
+provider "dynu" {
+  # Use DYNU_API_KEY environment variable by default.
+  # Set var.dynu_api_key in terraform.tfvars to override.
+  api_key = var.dynu_api_key
+}