Fix the bounds key path docs to indicate that additive animations are not supported. (#77)

Author: jverkoey

src/MDMAnimatableKeyPaths.h

@@ -58,7 +58,9 @@ FOUNDATION_EXPORT MDMAnimatableKeyPath MDMKeyPathBackgroundColor NS_SWIFT_NAME(b
  Equivalent UIView property: bounds
  Equivalent CALayer property: bounds
  Expected value type: CGRect or NSValue (containing a CGRect).
- Additive animation supported: Yes.
+ Additive animation supported: No.
+ TODO( https://github.com/material-motion/motion-animator-objc/issues/74 ):
+      Add support for additively animating CGRect types.
  */
 FOUNDATION_EXPORT MDMAnimatableKeyPath MDMKeyPathBounds NS_SWIFT_NAME(bounds);