Print a message before the compilation of bigger vocabulary namespaces

lib/rdf/vocabulary_namespace.ex

@@ -14,6 +14,7 @@ defmodule RDF.Vocabulary.Namespace do
   alias RDF.Utils.ResourceClassifier
 
   @vocabs_dir "priv/vocabs"
+  @big_vocab_threshold 300
 
   defmacro __using__(_opts) do
     quote do
@@ -34,6 +35,10 @@ defmodule RDF.Vocabulary.Namespace do
         {:data, data}   -> {rdf_data_vocab_terms(data, base_uri), data}
       end
 
+    if data && RDF.Data.subject_count(data) > @big_vocab_threshold do
+      IO.puts("Compiling vocabulary namespace for #{base_uri} may take some time")
+    end
+
     terms =
       terms
       |> term_mapping!(opts)