convert README volume

[coret]

The README states a shared volume has te be created. Although the shared volume is mentioned in the docker-compose.yaml file, the volume isn't used/referenced anywhere in the convert component. So for a working convert the shared volume is optional.

[coret]

The validator shows an example in which /opt/europeana_cc_lod_share/converted/centsprenten_edm_000001.ttl is used. Suggest to add conversion of the the censprenten dataset (is present in crawler README) to the convert README as an examples (then we'll have a complete pipeline example). Then the volume creation message also makes sense.