Docs for Reactive protocol

RxCocoa/Common/Reactive.swift

@@ -6,6 +6,28 @@
 //  Copyright © 2016 Krunoslav Zaher. All rights reserved.
 //
 
+/**
+We can use `Reactive` protocol as customization point for constrained protocol extensions.
+
+General pattern would be:
+
+```swift
+// 1. Conform SomeType to Reactive protocol
+extension SomeType: Reactive {}
+
+// 2. Extend Reactive protocol with constrain on Self
+// Read as: Reactive Extension where Self is a SomeType
+extension Reactive where Self: SomeType {
+    // 3. Put any specific reactive extension for SomeType here
+}
+```
+
+With this approach we can have more specialized methods and properties using 
+`Self` and not just specialized on common base type.
+
+See UIGestureRecognizer+Rx.swift as an example
+*/
+
 public protocol Reactive {
     
 }