Add the TODO item about implementing monotone?.

README.org

@@ -87,6 +87,12 @@ interaction with Org-mode.
 (define-pbf my-pbf (or (and (not :Go) :new-x) (and :Go :old-x)))
     #+END_SRC
 
+*** TODO Implement =monotone?=
+ =monotone?= would verify whether a given Boolean function is
+    monotone according to the definition in the book /Boolean
+    Functions: Theory, Algorithms, and Applications/ by Crama
+    and Hammer.
+
 *** TODO Split =networks= into general networks and threshold Boolean networks
 *** TODO Submit =update-graph= to =stchang=
 *** TODO Implement the BN \to RS conversion