sf2.2 note: _method override disabled by default

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.2
| Fixed tickets | n/a?

Just added a short note explaining that the `_method` hidden field method of overriding the HTTP method (method method method) is now disabled by default in Symfony 2.2, and must be explicitly enabled.