Add documentation for signals about django-smuggler

[jaap3]

What is the intended use of the save_data_on_filesystem? I'm not sure I understand.

[jaap3]

I would like to remove signals.py. They aren't used by smuggler itself and aren't documented well. Do you agree?

[semente]

If I remember well, it was done because someone would automatize the fixture generation but I don't remember how it was used. So I do agree in remove it. If later someone miss it we ask how it is used and decide if support it back.

Please, add a note in the backward-incompatible changes telling it is not available anymore. ;)