Create CONTRIBUTING.md, add link to CLA

CONTRIBUTING.md

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+We welcome contributions to the Photoshop Spaces project. In order to contribute to this project, we ask that you follow a few simple guidelines:
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+1. You must sign a [Contributor License Agreement](http://adobe.ly/1eIjOnF) and adhere to the guidelines in that document. This is for your protection as much as ours -- we want to make sure everything remains free and open, and that everyone gets the credit she or he deserves.
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+2. Code should conform to our [coding conventions](https://github.com/adobe-photoshop/spaces-design/wiki/Coding-Conventions), and must pass the JSHint and JSCS linters.
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+All PRs should be branched from master. All PRs will be reviewed by a core member of the team.