Link to instructions on how to install Cocoapods (#5)

diff --git a/README.md b/README.md
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@@ -12,8 +12,7 @@
 * Click <img src="http://gtms04.alicdn.com/tps/i4/TB1wCcqMpXXXXakXpXX3G7tGXXX-34-44.png" height="16" > (`Run` button)
 
 ### iOS
-* install `CocoaPods`.
-* Open `ios` directory, and then execute `pod install` in terminal.
+* [Install `CocoaPods`](https://guides.cocoapods.org/using/getting-started.html#installation).
+* Open `ios` directory, then execute `pod install` in terminal.
 * Open `WeexDemo.xcworkspace` with Xcode.
 * Click <img src="http://gtms04.alicdn.com/tps/i4/TB1wCcqMpXXXXakXpXX3G7tGXXX-34-44.png" height="16" > (`Run` button)
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