Update the README and add CONTRIBUTING.md

Update the README slightly with better organization/getting started steps. CONTRIBUTING.md is copied largely from Bootstrap.

Consider this a proposal - any policies/wording here you don't like? That's fine, leave a comment!

A couple specific changes:

 * Questions about how to use ember-table should be posted in Stack Overflow, not in the issue tracker. Consider adding "ember-table" to your list of watched tags on Stack Overflow.
 * [upcoming] Pull requests that add functionality should have tests for that functionality. This won't be required until we have a test framework in place.