Actual text for contribution guideline, renamed file to use .md

CONTRIBUTING

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-docs/internals/git-workflow.md

CONTRIBUTING.md

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+Contributing to Yii2
+====================
+
+Creating issues
+---------------
+
+- Please use English if possible.
+- Make sure it is clear how to reproduce it.
+- Make sure it is clear why you think it is necessary to fix.
+
+Contributing to the core framwork and official extensions
+---------------------------------------------------------
+
+- [Getting started with Yii2 development](docs/internals/getting-started.md)
+- [Git workflow for Yii 2 contributors](docs/internals/git-workflow.md)
+- [Translation workflow](docs/internals/translations.md)