Add decorator pattern

structural/decorator.md

@@ -0,0 +1,41 @@
+# Decorator Pattern
+Decorator structural pattern allows extending the function of an existing object dynamically without altering its internals.
+
+Decorators provide a flexible method to extend functionality of objects.
+
+## Implementation
+`LogDecorate` decorates a function with the signature `func(int) int` that 
+manipulates integers and adds input/output logging capabilities.
+
+```go
+type Object func(int) int
+
+func LogDecorate(fn Object) Object {
+	return func(n int) int {
+		log.Println("Starting the execution with the integer", n)
+
+		result := fn(n)
+
+		log.Println("Execution is completed with the result", result)
+
+        return result
+	}
+}
+```
+
+## Usage
+```go
+func Double(n int) int {
+    return n * 2
+}
+
+f := LogDecorate(Double)
+
+f(5)
+// Starting execution with the integer 5
+// Execution is completed with the result 10
+```
+
+## Rules of Thumb
+- Unlike Adapter pattern, the object to be decorated is obtained by **injection**.
+- Decorators should not alter the interface of an object.