Information about how to run specific tests in a class

My wording probably isn't the best but I think it would be helpful to explain that running something like

```
php codecept.phar  tests/acceptance/SignInCest.php:testLogin
```

...actually runs

```
SignInCest.php:testLogin
SignInCest.php:testLoginFails
SignInCest.php:testLoginSuccess
```

I think this is because it works how phpunit's --filter option works