mention negative section_thickness as per #43

This is a temporary fix until we implement something more substantive

README.md

@@ -121,7 +121,8 @@ Model.propagate_angles()
 #To reorder your sections according to the section numbers 
 Model.enforce_index_order()    
 #alternatively if you know the precise spacing (ie; 1, 2, 4, indicates that section 3 has been left out of the series) Then you can use      
-#Furthermore if you know the exact section thickness in microns this can be included instead of None        
+#Furthermore if you know the exact section thickness in microns this can be included instead of None
+#if your sections are numbered rostral to caudal you will need to specify a negative section_thickness      
 Model.enforce_index_spacing(section_thickness = None)
 #now we save which will produce a json file which can be placed in the same directory as your images and then opened with QuickNII. 
 Model.save_predictions(folderpath + 'MyResults')