schemas in doc directory

doc/schemas/ngram_parsing_flow.dot

+// dot ngram_parsing_flow.dot -Tpng -o ngram_parsing_flow.png
+digraph ngramflow {
+
+    edge [fontsize=10] ;
+    label=<<B><U>gargantext.util.toolchain</U></B><BR/>(ngram extraction flow)>;
+    labelloc="t" ;
+
+    "extracted_ngrams" -> "grouplist" ;
+    "extracted_ngrams" -> "occs+tfidfs" ;
+    "main_user_stoplist" -> "stoplist" ;
+    "stoplist" -> "mainlist" ;
+    "occs+tfidfs" -> "mainlist" [label="  TFIDF_LIMIT"];
+    "mainlist" -> "coocs" [label="  COOCS_THRESHOLD"] ;
+    "coocs" -> "specificity" ;
+
+    "specificity" -> "maplist" [label="MAPLIST_LIMIT\nMONOGRAM_PART"];
+
+    "maplist" -> "explore" ;
+    "grouplist" -> "maplist" ;
+}