Added instructions for contrubutors

Author: hiddentao

CONTRIBUTING.md

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+All examples in this repository get renedered to [waigojs.com/examples/](http://waigojs.com/examples/).
+
+To contribute an example to this repository, ensure the following:
+
+1. Your example is a sub-folder within the repository (like the other examples) and has a `README.md` file.
+2. Your `README.md` file has a H1 title which is a short descriptive name of your example.
+3. Your example's source code is contained in a `src/` sub folder.
+
+
+If you're not sure about any of the above then take a look at the existing examples for guidance.