added storage.engine note for UPGRADING to 0.6.0
added storage.engine note for UPGRADING to 0.6.0

OpenBEL/openbel-server,nbargnesi/openbel-server,OpenBEL/openbel-api,OpenBEL/openbel-api,OpenBEL/openbel-server,OpenBEL/openbel-api,nbargnesi/openbel-server,OpenBEL/openbel-server,nbargnesi/openbel-server,OpenBEL/openbel-api