The addition of the `TestRenderer.act()` method to the documentation broke the logical flow and "The returned instance has the following methods and properties." is now confusing since it's the `act()` method that is now documented afterwards. This commit aims at clarifying what is returned.
* Testing Docs
This expands the testing docs for reactjs.org. It adds 3 main docs -
- testing.md: An entry point to the testing docs
- testing-recipes.md: Common patterns when writing tests for React Components.
- testing-environments.md: Setting up your testing environment.
With help from @alexkrolick, @kentcdodds, @gaearon
