Added cookbook to show how to make a simple upload

| Q             | A
| ------------- | ---
| Doc fix?      | No
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #2346

This documentation refers to the ticket #2346, the second point saying:
```
2) Have a cookbook article that talks about uploading in Symfony, but nothing about Doctrine. This would include using a Form, validation, and moving the uploaded file in the controller.
```

Not sure if it was wise to put it in the cookbook/controller section. Feel free to tell me if I should put it elsewhere.
We try to avoid the first person perspective. So, this should be reworded. I suggest to use something like:

> Imaginge that you have a ``Product`` entity. Users should be able to upload files containing the description of those products.