Add assetic DIC tags reference.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.0+
| Fixed tickets | #2347

The one known issue is that the ``assetic.asset`` tag adds the asset to the asset manager, but AsseticBundle throws an exception when any asset has no associated formula. So at the moment this tag seems to be useless. I created an issue(symfony/AsseticBundle#183) in AsseticBundle. Once its filxed, I will add an example.