Readme fixes

- Update readme to use git init submodule recursive option
- Add database migration instruction
- Fix example PHP syntax

Also, not 100% sure about this one but I pretty sure if you set your form like this as in the exemple :

```php
echo $this->Form->input('PdfFile.file');
```

You'll have to use file file key after in the controller or model logic handling upload unless there is some callback magics happening (did not happened to me) :

```php
file_get_contents($this->data['PdfFile']**['file']**['tmp_name']
```