Create CONTRIBUTING.md

Author: nchaulet

CONTRIBUTING.md

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+## Issues
+
+Issues for node-geocoder library go here https://github.com/nchaulet/node-geocoder
+
+## Pull-Request and Code contributions
+
+Tests must pass, you can run the test using the command `npm test`
+Follow existing coding style, you can run jshint using `npm run lint`
+
+Thanks for contributing.