added README for unit tests

tests/README.md

+Yii 2.0 Unit tests
+==================
+
+DIRECTORY STRUCTURE
+-------------------
+
+      unit/                Unit tests to run with PHPUnit
+          data/            models, config and other test data
+              config.php   this file contains configuration for database and caching backends
+          framework/       the framework unit tests
+          runtime/         the application runtime dir for the yii test app
+      web/                 webapp for functional testing
+
+
+HOW TO RUN THE TESTS
+--------------------
+
+Make sure you have PHPUnit installed.
+
+Run PHPUnit in the yii repo base directory.
+
+```php
+phpunit
+```
+
+You can run tests for specific groups only:
+
+```php
+phpunit --group=mysql,base,i18n
+```
+
+You can get a list of available groups via `phpunit --list-groups`.
+
+TEST CONFIGURATION
+------------------
+
+PHPUnit configuration is in `phpunit.xml.dist` in repository root folder.
+You can create your own phpunit.xml to override dist config.
+
+Database and other backend system configuration can be found in `unit/data/config.php`
+adjust them to your needs to allow testing databases and caching in your environment.
\ No newline at end of file