add contributing.md with notes on how to contribute to the eak website

Bluntly stolen from the ember.js CONTRIBUTING.md
Merge pull request #459 from ghedamat/mattia/contributing

add contributing.md with notes on how to contribute to the eak website
Don’t call `factory()` before calling to `delegate()`. Allows injections to be specified in the provided `delegate()` function.
Merge pull request #462 from aexmachina/master

Don’t call `factory()` before calling to `delegate()` in `moduleFor()`