Add contribution guide

CONTRIBUTING.md

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+## json4s Contributers' Guide
+
+### Issues
+
+- Questions should be posted to [stackoverflow.com](http://stackoverflow.com/questions/tagged/json4s)
+- Please describe about your issue in detail (verison, situation, examples)
+- We may close your issue when we have no plan to take action right now. We appreciate your understanding.
+
+### Pull Requests
+
+- Pull requests basically should be sent toward "3.4" branch
+- Source/binary compatibility always must be kept as far as possible
+- Prefer creating scala source code for each class/object/trait (of course, except for sealed trait)
+- json4s build checks binary compatibility by using [MiMa](https://github.com/typesafehub/migration-manager/wiki/Sbt-plugin) for maintenance releases (e.g. 3.3.x).
+
+#### Branches
+
+##### 3.4 (the default branch)
+
+- The latest stable version
+- This branch must be able to build against Scala 2.10 and 2.11
+
+##### 3.3
+
+- The version 3.3 series maintainance branch
+- All the backports must be source/binary compatibility
+- This branch must be able to build against Scala 2.10 and 2.11
+
+#### Testing your pull request
+
+All the pull requests must pass the Travis CI jobs before merging them.
+
+https://travis-ci.org/json4s/json4s
+
+Testing with default settings is required when push changes:
+
+```sh
+sbt test
+```