add a new 'overview' MVC example

Former-commit-id: f73cbf6010595c639f6c5b5119e2ec41bc9802a5

_examples/mvc/overview/environment/environment.go

@@ -0,0 +1,10 @@
+package environment
+
+// Env is the environment type.
+type Env string
+
+// Available environments example.
+const (
+	PROD Env = "production"
+	DEV  Env = "development"
+)