Add CONTRIBUTING

Author: kemitchell

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+Pull requests are most welcome!
+
+Please:
+
+1. Use [JavaScript Standard Style](https://www.npmjs.com/packages/standard).
+
+2. Add tests to the code blocks in `README.md`, using Node.js' built-in
+   `assert` module. Those examples are run as the test suite.
+
+3. Add yourself to `AUTHORS`.
+
+4. License your contributions under the existing, MIT-style license.