added comment

Chapter07/in_region_cost_example.py

@@ -4,6 +4,7 @@ import skimage.io
 import slgbuilder
 
 # This is to fix deprecated alias in numpy, but still used in slgbuilder
+# You need to add it to any script using (this version of) slgbuilder
 np.bool = bool
 np.int = int