Add Pinchable documentation.

Summary: Part of #8

Reviewers: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Reviewed By: O4 Material Motion Apple platform reviewers, O2 Material Motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D1820

src/Pinchable.swift

@@ -18,19 +18,30 @@ import UIKit
 import MaterialMotionRuntime
 
 /**
- A plan that enables a target to be scaled by pinching.
+ Adds the scale delta from a pinch gesture recognizer to the target transform's scale using
+ multiplication.
+
+ Supported target types: UIView.
 
  Enables isUserInteractionEnabled on the target view.
  */
 @objc(MDMPinchable)
 public final class Pinchable: NSObject, Plan {
+  /** The gesture recognizer whose events will be observed. */
   public let pinchGestureRecognizer: UIPinchGestureRecognizer
+
+  /**
+   Whether or not the target's anchorPoint should be modified each time the gesture
+   recognizer begins.
+   */
   public var shouldAdjustAnchorPointOnGestureStart = true
 
+  /** Creates a plan with a new pinch gesture recognizer. */
   public convenience override init() {
     self.init(withGestureRecognizer: UIPinchGestureRecognizer())
   }
 
+  /** Events generated by the provided gesture recognizer will be observed. */
   public init(withGestureRecognizer recognizer: UIPinchGestureRecognizer) {
     self.pinchGestureRecognizer = recognizer
     super.init()