adds usecase description for harvester

usecases/collections_harvester.md

+
+```plantuml
+@startuml
+object catalog {
+    ...
+    <dcat:Dataset rdf:about="uri_collection">
+    \t<dct:type rdf:resource="http://dcat-ap.de/def/datasetTypes/collection" />
+    \t<dct:hasVersion rdf:resource="uri1" />
+    \t<dct:hasVersion rdf:resource="uri2" />
+    </dcat:Dataset>
+
+    ...
+    <dcat:Dataset rdf:about="uri1">
+    \t<dct:isVersionOf rdf:resource="uri_collection" />
+    \t...
+    </dcat:Dataset>
+
+    ...
+    <dcat:Dataset rdf:about="uri2">
+    \t<dct:isVersionOf rdf:resource="uri_collection" />
+    \t...
+    </dcat:Dataset>
+}
+note right
+    Falls Collection uri_collection nicht existiert:
+        Erzeuge Collection
+    Füge alle Datasets mit isVersionOf uri_collection
+        zu Collection hinzu
+end note
+
+@enduml
+```
+
+
+```plantuml
+@startuml
+object catalog {
+    ...
+    <dcat:Dataset rdf:about="uri_collection">
+    \t<dct:type rdf:resource="http://dcat-ap.de/def/datasetTypes/collection" />
+    \t<dct:hasVersion rdf:resource="uri1" />
+    \t<dct:hasVersion rdf:resource="uri2" />
+    </dcat:Dataset>
+
+    ...
+    <dcat:Dataset rdf:about="uri1">
+    \t<dct:isVersionOf rdf:resource="uri_collection" />
+    \t...
+    </dcat:Dataset>
+
+    ...
+    <dcat:Dataset rdf:about="uri2">
+    \t<dct:isVersionOf rdf:resource="uri_collection" />
+    \t...
+    </dcat:Dataset>
+}
+note right
+    Falls Collection uri_collection nicht existiert:
+        Erzeuge Collection
+    Füge alle Datasets mit isVersionOf uri_collection
+        zu Collection hinzu
+end note
+
+@enduml
+```