Install instructions for Composer

The instructions tell to run codeception as if it was a PHP script or PHAR .
```bash
php vendor/bin/codecept
```
When you take a look into the vendor/bin directory you will not find any PHP code.
Instead the composer package comes with *nix shell scripts and Windows batch files for both codeception and phpunit.

That won't work! PHP can neither run shell scripts nor batch files!
I stripped the php in front and everything works as expected!