Improve local dev override docs and add read-only example

examples/read_only/terraform.tfvars.example

@@ -0,0 +1,6 @@
+# Copy to terraform.tfvars and adjust values for your environment.
+# dynu_api_key can be omitted when DYNU_API_KEY is exported in your shell.
+# dynu_api_key = "replace-with-dynu-api-key"
+
+# Use a hostname that exists in your Dynu account.
+hostname = "www.example.com"