[Slider] Stateful API Section Added. (#3565)

Pivotal: https://www.pivotaltracker.com/story/show/156898304

components/Slider/README.md

@@ -117,6 +117,32 @@ func didChangeSliderValue(senderSlider:MDCSlider) {
 ```
 <!--</div>-->
 
+### Stateful API
+
+`MDCSlider` exposes stateful APIs to customize the colors for different control states. In order to use this API you must enable `statefulAPIEnabled` on your `MDCSlider` instances.
+
+<!--<div class="material-code-render" markdown="1">-->
+#### Swift
+
+``` swift
+ let slider = MDCSlider()
+ slider.isStatefulAPIEnabled = true
+
+ // Setting a thumb color for selected state.
+ slider.setThumbColor(.red, for: .selected)
+```
+
+#### Objective C
+
+``` objc
+ MDCSlider *slider = [[MDCSlider alloc] init];
+ slider.statefulAPIEnabled = YES;
+
+ // Setting a thumb color for selected state.
+ [slider setThumbColor:[UIColor redColor] forState:UIControlStateSelected];
+```
+<!--</div>-->
+
 ### The differences between the UISlider class and the MDCSlider class:
 
 Does not have api to: