Add Contributing Guidelines

Author: ogaclejapan

CONTRIBUTING.md

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+# Contributing
+
+If you would like to contribute code to SmartTabLayout you can do so through GitHub by
+forking the repository and sending a pull request.
+
+When submitting code, please make every effort to follow existing conventions
+and style in order to keep the code as readable as possible.
+
+## Coding Style
+
+* Use the AndroidModernStyle of [Android Code Styles](https://github.com/ogaclejapan/android-code-styles) repository