Updated Usage with Create React App - avoiding #71 (#72)

Using jest-enzyme@3.0.0+ with Create React App is causing "TypeError: expect.addSnapshotSerializer is not a function" error.
This is happening because jest-enzyme@3.0.0+ depends on jest@19.0.0 while Create React App is currently using jest@18.1.0.
Added README warning - pending to be modified when Create React App updates their Jest dependency.

See also #71.

packages/jest-enzyme/README.md

@@ -50,3 +50,9 @@ If you are using Create React App, instead of adding to your `package.json` as a
  ```
 
  This is documented on Create React App at the bottom of the [Testing Components](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#testing-components) section. There is also more information about [Initializing Test Environment](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#initializing-test-environment).
+
+ Also, instead of installing the latest version you should stick with versions below 3, because Create React App currently [uses](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/package.json#L50) a version of jest which will cause [this issue](https://github.com/blainekasten/enzyme-matchers/issues/71):
+
+```
+yarn add jest-enzyme@2.1.2 --dev
+```