iOS playground seperation

ios/README.md

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+## The submodule weex-playground
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+[Weex Playground](https://github.com/apache/incubator-weex-playground/tree/master/ios) can be used to test and preview weex pages on iOS and it has been added as a submodule.
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+### Relationship between weex and playground
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+- The playground submodule will be cloned and updated automatically when developers clone the ‘incubator-weex’ repo and build it. In this case, playground project will use the current incubator-weex project as it's pod source.
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+- When developer directly clone the 'weex-playground' repo and run it separately, the playground project will use the lastest incubator-weex project in master branch as it's pod source.
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