Improve local dev override docs and add read-only example

examples/read_only/variables.tf

@@ -0,0 +1,12 @@
+variable "dynu_api_key" {
+  description = "Dynu API key. Leave null to use DYNU_API_KEY from environment."
+  type        = string
+  default     = null
+  sensitive   = true
+}
+
+variable "hostname" {
+  description = "Fully-qualified hostname used by dynu_domain and dynu_dns_records data sources."
+  type        = string
+  default     = "www.example.com"
+}