coq-cut: add reference to exported files

ulo/coq-cut.sh

 #! /bin/sh
 
-# cut out some components from the rdf files exported from Coq
+# cut out some components from the rdf files exported from Coq [1]
 # that GraphDB does not like; right now I don't know if they
 # are important or if they can be skipped, but I just want a working
 # prototype first of all
+#
+# [1] https://gl.mathhub.info/Coqxml
 
 set -eu