add contribution guide

CONTRIBUTING.md

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+# Contributing to Waterline
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+Waterline follows the [Sails Contribution Guide](https://github.com/balderdashy/sails/blob/master/CONTRIBUTING.md).
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+The Contribution Guide is designed to help you get off the ground quickly contributing to Waterline. Reading it thoroughly will help you write useful issues, make eloquent proposals, and submit top-notch code that can be merged quickly. Respecting the guidelines laid out in the guide helps make the core maintainers of Waterline more productive, and makes the experience of working with Waterline positive and enjoyable for the community at large.
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+If you are working on a pull request, **please carefully read this file from top to bottom**. In case of doubt, open an issue in the issue tracker or contact someone from our [core team](https://github.com/balderdashy/sails#team) on Twitter. Especially do so if you plan to work on something big. Nothing is more frustrating than seeing your hard work go to waste because your vision does not align with planned or ongoing development efforts of the project's maintainers.
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