added assumption

hm-inference/README.md

@@ -27,6 +27,9 @@ User can define functions/types by adding them to the prelude.
 Note that variables can be used before definition for now, we would do another
 pass after type checking to prevent this. The pass would also check for return.
 
+We assume that class type do not contain any unbound type variables. (methods
+can contain unbound type variables, if the variable is bound to the class)
+
 ## TODO
 - Parse class/function definition.
 - Occur check to prevent infinite function types.
@@ -45,4 +48,3 @@ pass after type checking to prevent this. The pass would also check for return.
   relationship, check whether the type variable can be instantiated to a certain
   type.
 
-