Hardening/136

Implements the issue https://github.com/telefonicaid/fiware-connectors/issues/136

Additionally, minor modifications have been done in the main README regarding missing Flume event headers.
Much clearer with a few <i>that</i>:
  o (that it is a POST request, that the target is 'notify' and that the headers are OK)
  o (that it is in Json format)