Add keyframe guide.

Summary: Fixes #7

Reviewers: O2 Material Motion, featherless

Reviewed By: O2 Material Motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2084

README.md

@@ -114,6 +114,7 @@ To run all unit tests, run the following commands:
 ## Guides
 
 1. [How to animate a property with a Tween plan](#how-to-animate-a-property-with-a-tween-plan)
+1. [How to animate a property with a keyframe Tween plan](#how-to-animate-a-property-with-a-keyframe-tween-plan)
 
 ### How to animate a property with a Tween plan
 
@@ -122,6 +123,17 @@ Tween tween = new Tween<>(property, duration, from, to);
 runtime.addPlan(tween, view);
 ```
 
+### How to animate a property with a keyframe Tween plan
+
+```java
+float[] values = new float[] {0f, .5f, 1f};
+float[] offsets = new float[] {0f, .25f, 1f};
+
+Tween tween = new Tween<>(property, duration, values);
+tween.offsets = offsets;
+runtime.addPlan(tween, view);
+```
+
 ## Contributing
 
 We welcome contributions!