Add a warning message on directed graphs.

src/dodb/directed_graph.cr

@@ -3,6 +3,8 @@ require "json"
 
 require "./indexer.cr"
 
+# WARNING: this code hasn't been reviewed nor used in years.
+
 class DODB::DirectedGraph(V) < DODB::Indexer(V)
 	property name         : String
 	property key_proc     : Proc(V, Array(String))