Add ngsi2cosmos migration path to Cygnus in documentation

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/105
My plan was to include that kind of information in specific documents talking about moving between different Cygnus versions. I mean, this document is about moving from ngsi2cosmos to Cygnus; if a version higher than 0.1 is needed, then not only a ngsi2cosmos to Cygnus migration is need but a "intra-cygnus" migration as well.

Consequently, my final aim is to replace the "Please log into Cosmos and run the script available at `resources/cygnus_translators/cygnus-translator-0.1-to-0.2.sh`" with a "Plase, go to this other document in order to know how to do such data translation".

In addition, having those different documents avoids a non ngsi2cosmos (but Cygnus 0.1) user to read about this ngsi2cosmos-related document.