Add Singletons page (#36)

* Create Singletons

* Update README.md

* Fix singleton name

* PR feedback

README.md

@@ -16,6 +16,7 @@ The Swift coding conventions guide documents many best practices for writing Swi
 * [Naming](Naming.md)
 * [Property Observers](PropertyObservers.md)
 * [Range Operators](RangeOperators.md)
+* [Singletons](Singletons.md)
 * [Type Casting](TypeCasting.md)
 * [Type Inference](TypeInference.md)
 

Singletons.md

@@ -0,0 +1,39 @@
+# Singletons
+
+## Convention
+
+If a singleton must be used (and effort should be taken to avoid this where possible), then they should always be implemented as a class with a shared reference to an instance of itself, with instance properties, rather than static properties.
+
+## Rationale
+
+Following this convention makes it clear that this class is a singleton and should be used that way. More importantly, it makes testing significantly simpler as new versions can be instatiated and passed around. 
+
+## Examples
+
+```swift
+// good: The singleton has no static properties
+class MyManager {
+  static let shared = MyManager()
+  var someVar: String = "Hello"
+}
+
+// bad: The singleton only has static properties
+class MyManager {
+  static var someVar: String = "Hello"
+}
+
+// A more complete example
+public class OurDefaults {
+  // Called throughout the application
+  public static let shared = OurDefaults(userDefaults: .standard)
+
+  private let userDefaults: UserDefaults
+
+  // Called during test case set up
+  // Initializer also makes it clear how many dependencies this class has.
+  init(userDefaults: UserDefaults) {
+    self.userDefaults = userDefaults
+  }
+
+}
+```