Add simple contributing guidelines

CONTRIBUTING.md

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+# Contributing
+
+We are open to, and grateful for, any contributions made by the community.
+
+## Reporting issues and asking questions
+
+Before opening an issue, please search the [issue tracker](https://github.com/erikras/redux-form/issues) to make sure your issue hasn’t already been reported.
+
+**We use the issue tracker to keep track of bugs and improvements** to Redux Form itself, its examples, and the documentation. We encourage you to open issues to discuss improvements, architecture, internal implementation, etc. If a topic has been discussed before, we will ask you to join the previous discussion.
+
+For support or usage questions, please search and ask on [StackOverflow with a `redux-form` tag](https://stackoverflow.com/questions/tagged/redux-form). We ask you to do this because StackOverflow has a much better job at keeping popular questions visible. Unfortunately good answers get lost and outdated on GitHub.
+
+## Sending a pull request
+
+For non-trivial changes, please open an issue with a proposal for a new feature or refactoring before starting on the work. We don’t want you to waste your efforts on a pull request that we won’t want to accept.
+
+Please try to keep your pull request focused in scope and avoid including unrelated commits.
+
+After you have submitted your pull request, we’ll try to get back to you as soon as possible. We may suggest some changes or improvements.