Merge pull request postcss#63 from postcss/contributing-md

Add CONTRIBUTING.md

Author: andyjansson

CONTRIBUTING.md

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+# Contributing
+
+The following outlines a set of guidelines for contributing to this project. These are mostly guidelines, not rules. Use your best judgement, and feel free to propose changes to this document in a pull request.
+
+## Asking questions
+
+Prefer asking questions in the [PostCSS chat](https://gitter.im/postcss/postcss) over opening issues.
+Leveraging the community would likely get you an answer faster, as well as leave us available to solve issues and make the product better. 
+
+## Reporting Bugs
+
+Before submitting a new issue, be sure to make a cursory search to see if the problem has already been reported. If it has and the issue is still open, leave a comment on the existing issue instead of opening a new one. 
+
+If no prior issue exist, create a new one. Explain the problem and include additional details to help maintainers reproduce the problem:
+
+* Use a clear and descriptive title.
+* Summarize the problem you are having.
+* Include details about your configuration and environment.
+* Provide steps to reproduce the problem.
+* Describe what behavior you expected to see.
+
+## Requesting Enhancements
+
+Before submitting a new issue, be sure to make a cursory search to see if the enhancement has already been requested. If no such issue exists, create a new one and detail it to the best of your ability: 
+
+* Use a clear and descriptive title.
+* Provide a description of the proposed enhancement. 
+* Exemplify what needs/uses this enhancement would address. 
+
+## Submitting Pull Requests
+
+Before contributing any code to the project, be sure to either open a new issue in the issue tracker detailing what you intend to contribute, or comment on an existing issue if one exists.
+This allows us to:
+
+* give feedback early on before significant effort has been put into the endevour. 
+* align your contribution with ongoing efforts. 
+* make sure that there's no ongoing effort into the issue already. 
+
+When submitting your pull request, make sure that you: 
+
+* use a clear and descriptive title. 
+* summarize your contribution.  
+* list the issues that this contribution addresses. 
+* include tests for your contribution.