add docstring

src/coscine/metadata.py

@@ -688,6 +688,13 @@ class QueryResult:
         Computes the union between a list of files, a file (coscine.FileObject)
         property and the property reflected in the SPARQL query result
         identified via the column it appears in.
+
+        Example
+        -------
+        result = resource.query(r"SELECT ?path ?p ?o WHERE { ?path ?p ?o . }")
+        files = resource.files(recursive=True)
+        for f in result.union(files, coscine.FileObject.uri, "path"):
+            print(f.name)
         """
         result = []
         column_index = self.headers.index(column)