Add Test section in README.md

:panda_face: 
I would rather show baby steps on running the tests (including installing composer), for example:
### Linux

```sh
$ cd <project folder>
$ curl -sS https://getcomposer.org/installer | php
$ composer.phar install
$ vendor/bin/phpunit tests
```

Above, we install composer and then the library dependencies (which includes Zend and Symfony validation libraries as well as PHPUnit). On the last command, we run the PHPUnit test suite present in the `tests` directory.