Add note to readme about references_uuid dependencies

[stpaultim]

The references_uuid sub module included in references is dependent upon the UUID module.

The UUID module does not have a stabile release yet. Should we either:

1. Try to get a stabile release of the UUID module (Status? Is this ready for release uuid#2)
   or
2. Put a note in the README clarifying the situation and alerting users to the situation.