Adds testing section (#687)

Integrates comments by @langpavel

docs/README.md

@@ -6,6 +6,7 @@
 * [React Style Guide](./react-style-guide.md)
 * [How to configure text editors and IDEs](./how-to-configure-text-editors.md)
 * [Data fetching with WHATWG Fetch](./data-fetching.md)
+* [Testing your application](./testing-your-application.md)
 
 ### Questions
 

docs/testing-your-application.md

@@ -0,0 +1,97 @@
+## Testing your application
+
+### Used libraries
+
+RSK comes with the following libraries for testing purposes:
+
+- [Mocha](https://mochajs.org/) - Node.js and browser test runner
+- [Chai](http://chaijs.com/) - Assertion library
+- [Enzyme](https://github.com/airbnb/enzyme) - Testing utilities for React
+
+You may also want to take a look at the following related packages:
+
+- [jsdom](https://github.com/tmpvar/jsdom)
+- [react-addons-test-utils](https://www.npmjs.com/package/react-addons-test-utils)
+
+### Running tests
+
+To test your application simply run the
+[`npm test`](https://github.com/kriasoft/react-starter-kit/blob/b22b1810461cec9c53eedffe632a3ce70a6b29a3/package.json#L154)
+command which will:
+- recursively find all files ending with `.test.js` in your `src/` directory
+- mocha execute found files
+
+```bash
+npm test
+```
+
+### Conventions
+
+- test filenames MUST end with `test.js` or `npm test` will not be able to detect them
+- test filenames SHOULD be named after the related component (e.g. create `Login.test.js` for
+`Login.js` component)
+
+### Basic example
+
+To help you on your way RSK comes with the following
+[basic test case](https://github.com/kriasoft/react-starter-kit/blob/master/src/components/App/App.test.js)
+you can use as a starting point:
+
+```js
+import React from 'react';
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+import App from './App';
+
+describe('App', () => {
+
+  it('renders children correctly', () => {
+    const wrapper = shallow(
+      <App context={{ insertCss: () => {} }}>
+        <div className="child" />
+      </App>
+    );
+
+    expect(wrapper.contains(<div className="child" />)).to.be.true;
+  });
+
+});
+```
+
+### React-intl example
+
+React-intl users MUST render/wrap components inside an IntlProvider like the example below:
+
+The example below example is a drop-in test for the RSK `Header` component:
+
+```js
+import React from 'react';
+import Header from './Header';
+import IntlProvider from 'react-intl';
+import Navigation from '../../components/Navigation';
+
+describe('A test suite for <Header />', () => {
+
+  it('should contain a <Navigation/> component', () => {
+    it('rendering', () => {
+      const wrapper = renderIntoDocument(<IntlProvider locale="en"><Header /></IntlProvider>);
+      expect(wrapper.find(Navigation)).to.have.length(1);
+    });
+  });
+
+});
+```
+
+Please note that  NOT using IntlProvider will produce the following error: 
+
+> Invariant Violation: [React Intl] Could not find required `intl` object. <IntlProvider>
+> needs to exist in the component ancestry.
+
+### Linting
+
+Running RSK eslint will also scan your test files:
+
+```bash
+npm run eslint
+```
+