update readme for iOS build instruments

README.md

@@ -119,7 +119,12 @@ To compile, open *`Pal.xcodeproj`* with `Xcode`, and click Build. You need to ha
 iOS
 ---
 
-To compile, open the project *`ios/SDLPal/SDLPal.xcodeproj`* with `Xcode`, and click Build.
+To compile, please first install dependencies via CocoaPods following the above instruments( only need do it once ), then open the project *`ios/SDLPal/SDLPal.xcworkplace`* with `Xcode`, and click Build.
+```shell
+$ cd iOS/SDLPAL
+$ sudo gem install cocoapods
+$ pod install
+```
 
 Android
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