Add WebIDL summary and examples. Closes #15.

Please review @heycam @marcoscaceres. Also feel free to bring others into this that are writing specs with promises.

Rendered diff [here](https://github.com/w3ctag/promises-guide/pull/21/files?short_path=04c6e90#diff-04c6e90faac2675aa89e2176d2eec7d8)