Update README.md to include testing gotchas (#136)

Author: codegoblin

Diff for: README.md

@@ -126,6 +126,11 @@ var App = React.createClass({
 
 ReactDOM.render(<App/>, appElement);
 ```
+# Testing
+
+When using React Test Utils with this library, here are some things to keep in mind:
+- You need to set isOpen={true} on the modal component for it to render its children.
+- You need to use the `.portal` property, as in `ReactDOM.findDOMNode(renderedModal.portal)` or `TestUtils.scryRenderedDOMComponentsWithClass(Modal.portal, 'my-modal-class')` to acquire a handle to the inner contents of your modal.
 
 By default the modal is closed when clicking outside of it (the overlay area). If you want to prevent this behavior you can
 pass the 'shouldCloseOnOverlayClick' prop with 'false' value.