Add ngsi2cosmos migration path to Cygnus in documentation

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/105
Maybe we should provide a little more detail on how use the script, eg. example command line to use it. In addition, I think we should include a "preparation" step, including:

1. Checklist regardig things to check before starting. E.g. how much local space we need in the VM where the script runs as a function of the data to migrate? (this is just an example... actually I don't remember it the script does thinks in local :)
2. Menction (and stress) the need of backup existing data, given that in the case a repentine fail occurs (network fail, VM reboot, etc.) the data could end in an inconsisten state (or I'm wrong?).