Add documentation for shallow testing

docs/docs/10.4-test-utils.md

@@ -130,3 +130,46 @@ ReactComponent findRenderedComponentWithType(ReactComponent tree, function compo
 ```
 
 Same as `scryRenderedComponentsWithType()` but expects there to be one result and returns that one result, or throws exception if there is any other number of matches besides one.
+
+
+## Shallow rendering
+
+Shallow rendering is an experimental feature that lets you render a component "one level deep" and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered. This does not require a DOM.
+
+```javascript
+ReactShallowRenderer createRenderer()
+```
+
+Call this in your tests to create a shallow renderer. You can think of this as a "place" to render the component you're testing, where it can respond to events and update itself.
+
+```javascript
+shallowRenderer.render(ReactElement element)
+```
+
+Similar to `React.render`.
+
+```javascript
+ReactComponent shallowRenderer.getRenderOutput()
+```
+
+After render has been called, returns shallowly rendered output. You can then begin to assert facts about the output. For example, if your component's render method returns:
+
+```javascript
+<div>
+  <span className="heading">Title</span>
+  <Subcomponent foo="bar" />
+</div>
+```
+
+Then you can assert:
+
+```javascript
+result = renderer.getRenderOutput();
+expect(result.type).toBe('div');
+expect(result.props.children).toEqual([
+  <span className="heading">Title</span>
+  <Subcomponent foo="bar" />
+]);
+```
+
+Shallow testing currently has some limitations, namely not supporting refs. We're releasing this feature early and would appreciate the React community's feedback on how it should evolve.