Add snapshot and update podspec.

README.md

@@ -7,18 +7,40 @@
 
 Yet another drop-in replacement of `UIPageViewController`, inspired by Apple's offical Music app.
 
+## What problem does YTPageController try to resolve?
+
+YTPageController introduces a general solution to achieve a smooth transition when user scrolls between view controllers, just as what Apple did in their Music app:
+
+To implement this effect, just add these lines of code in your `YTPageControllerDelegate`:
+
+```objectivec
+- (void)pageController:(YTPageController *)pageController willTransitionToIndex:(NSInteger)index {
+    [pageController.pageCoordinator animateAlongsidePagingInView:self.segmentedControl animation:^(id<YTPageTransitionContext>  _Nonnull context) {
+        // Update your segmented control according to the information contained in YTPageTransitionContext
+        self.segmentedControl.selectedSegmentIndex = [context toIndex];
+    } completion:^(id<YTPageTransitionContext>  _Nonnull context) {
+        if ([context isCanceled]) {
+            // Revert to original state if transition canceled
+            self.segmentedControl.selectedSegmentIndex = [context fromIndex];
+        }
+    }];
+}
+```
 
+The key idea to control the percent of its animation is to set the CALayer's `speed` property to 0, and then update the `timeOffset` value according to UIScrollView's `contentOffset`. In addition to `UISegmentedControl`, you can also animate `UITabBar` or any `UIView` subclass you like. Refer to the example project for detailed information.
 
 ## Example
 
 To run the example project, clone the repo, and run `pod install` from the Example directory first.
 
 ## Requirements
 
+iOS 8.0 or above.
+May be working from iOS 6.0, but I didn't test it.
+
 ## Installation
 
-YTPageController is available through [CocoaPods](http://cocoapods.org). To install
-it, simply add the following line to your Podfile:
+YTPageController is available through [CocoaPods](http://cocoapods.org). To install it, simply add the following line to your Podfile:
 
 ```ruby
 pod "YTPageController"
@@ -33,3 +55,4 @@ Yeatse CC, [email protected]
 YTPageController is available under the MIT license. See the LICENSE file for more info.
 
 
+

YTPageController.podspec

@@ -10,7 +10,7 @@ Pod::Spec.new do |s|
 #   * Finally, don't worry about the indent, CocoaPods strips it!
 
   s.description      = <<-DESC
-Another drop-in replacement for UIPageViewController. You can run animations alongside the paging process. Fully storyboard support.
+  YTPageController introduces a general solution to archieve a smooth transitoin when user scrolls between view controllers, just as what Apple did in their Music App. With the help of YTPageController, you can implement the effect in a few lines.
                        DESC
 
   s.homepage         = 'https://github.com/yeatse/YTPageController'