Merge pull request #58 from maria-simon/master

Adding CONTRIBUTING.md

Author: gnongsie

CONTRIBUTING.md

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++ Thanks for contributing to the Authorize.Net Node SDK.
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++ Before you submit a pull request, we ask that you consider the following:
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+     - Submit an issue to state the problem your pull request solves or the funtionality that it adds. We can then advise on the feasability of the pull request, and let you know if there are other possible solutions.
+     - Part of the SDK is auto-generated based on the XML schema. Due to this auto-generation, we cannot merge contributions for request or response classes. You are welcome to open an issue to report problems or suggest improvements. Auto-generated classes are inside [sdk-node/lib/apicontracts.js](https://github.com/AuthorizeNet/sdk-node/tree/master/lib) and [sdk-node/lib/apicontrollers.js](https://github.com/AuthorizeNet/sdk-node/tree/master/lib) folders, except [sdk-node/lib/apicontrollersbase.js](https://github.com/AuthorizeNet/sdk-node/tree/master/lib).
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