Security voters reference in @security doc page

Added more information about the security annotation and how to create new security voters.
I don't think that the `ref` role will work this way. You should use something like this in the text:

```rst
[...] please refer to `the Security Voters page`_.
```

And then add the end of the document, you add the referenced URL:

```rst
.. _`the Security Voters page`: http://symfony.com/doc/current/cookbook/security/voters_data_permission.html
```