updated codeception docs. fixes #1558

tests/README.md

@@ -11,6 +11,13 @@ To run the tests, follow these steps:
    - `functional.suite.yml` for [functional testing](http://codeception.com/docs/05-FunctionalTests) and
      `unit.suite.yml` for [unit testing](http://codeception.com/docs/06-UnitTests) should already work out of the box
      and should not need to be adjusted.
+   - If you want to run acceptance tests, you need to download [selenium standalone](http://www.seleniumhq.org/download/)
+     and start it with command `java -jar {selenium-standalone-name}.jar`.
+     After that you can use `WebDriver` codeception module that will connect to selenium and launch browser.
+     This also allows you to use [Xvfb](https://en.wikipedia.org/wiki/Xvfb) in your tests which allows you to run tests
+     without showing the running browser on the screen. There is codeception [blog post](http://codeception.com/05-24-2013/jenkins-ci-practice.html)
+     that explains how it works.
+
 3. Go to the application base directory and build the test suites:
    ```
    php codecept.phar build    // rebuild test scripts, only need to be run once