Added contributing document

CONTRIBUTING.md

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+Contributing
+---
+
+The issue tracker is at [github.com/CartoDB/torque](https://github.com/CartoDB/torque).
+
+We love pull requests from everyone, see [Contributing to Open Source on GitHub](https://guides.github.com/activities/contributing-to-open-source/#contributing).
+
+
+## Submitting Contributions
+
+* You will need to sign a Contributor License Agreement (CLA) before making a submission. [Learn more here](https://carto.com/contributions).