Create CONTRIBUTING.md

Author: wkh237

CONTRIBUTING.md

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+For developers who interested in making contribution to this project, please see [https://github.com/wkh237/react-native-fetch-blob-dev](https://github.com/wkh237/react-native-fetch-blob-dev) for more information.
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+Please read the following rules before opening a PR :
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+1. If the PR is offering a feature please make the PR to our "Feature Branch" 0.11.0
+2. Bug fix request to "Bug Fix Branch" 0.10.6
+3. Correct README.md can directly to master