Add example of usage for linking permission and modifiers

[mcourtot]

This is not obvious from the current documentation.

[esthervanenckevort]

This is also relevant for the FAIR Genomes project. In our current model we have a 'is restricted by' (DUO:00000010) predicate between the instance of a Data Use Permission and the instance of a Data Use Modifier, but the current definition of DUO:00000010 doesn't allow that, even though the examples suggest it should. See also issue #104.