Added readme about tests.

tests/README.md

@@ -0,0 +1,15 @@
+This folder contains various tests for the basic application.
+These tests are developed with [Codeception PHP Testing Framework](http://codeception.com/).
+
+To run the tests, follow these steps:
+
+1. [Install Codeception](http://codeception.com/quickstart) if you do not have it yet.
+2. Create test configuration files based on your environment:
+   - Copy `acceptance.suite.dist.yml` to `acceptance.suite.yml` and customize it;
+   - Copy `functional.suite.dist.yml` to `functional.suite.yml` and customize it;
+   - Copy `unit.suite.dist.yml` to `unit.suite.yml` and customize it.
+3. Switch to the parent folder and run tests:
+
+```
+php codecept.phar run
+```