insert new rule to end of rules
if (rules == NULL) {rules = new_rule;} // if rules == NULL, new_rule becomes first rule
            else { // if rules != NULL
                Rule *curr = rules; // want to iterate to end of rules
                while (curr->next_rule != NULL) {
                    curr = curr->next_rule;
                } // curr->next_rule == NULL
                curr->next_rule = new_rule; // insert new rule to end of rules
            }