Add ngsi2cosmos migration path to Cygnus in documentation

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/105
In case an additional document deals with that part and that document not being part of this PR, I will add the following after "Please log into Cosmos and run the script available at resources/cygnus_translators/cygnus-translator-0.1-to-0.2.sh": "(TBD: the details will be provided in an additional document, which link will be included here)" or something like that.