Added a CONTRIBUTING.md file

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+## Contributing
+
+Contributions to the `jsoneditor` library are very welcome! We can't do this
+alone. You can contribute in different ways: spread the word, report bugs, come
+up with ideas and suggestions, and contribute to the code.
+
+There are a few preferences regarding code contributions:
+
+- `jsoneditor` follows the node.js code style as described
+  [here](http://nodeguide.com/style.html).
+- Send pull requests to the `develop` branch, not the `master` branch.
+- Only commit changes done in the source files under `src`, not to the builds
+  which are located in the root of the project (like `jsoneditor.js`,
+  `jsoneditor.css`, etc.
+
+Thanks!
+