Add singleton creational pattern

creational/singleton.md

@@ -0,0 +1,35 @@
+#Singleton Pattern
+Singleton creational design pattern restricts the instantiation of a type to a single object.
+
+## Implementation
+```go
+package singleton
+
+type singleton map[string]string
+
+var once sync.Once
+
+var instance *singleton
+
+func New() *singleton {
+	once.Do(func() {
+		instance = make(singleton)
+	})
+
+	return instance
+}
+```
+
+## Usage
+```go
+s := singleton.New()
+
+s["this"] = "that"
+
+s2 := singleton.New()
+
+// s2["this"] == "that"
+```
+
+## Rules of Thumb
+- Singleton pattern represents a global state and most of the time reduces testability.