Update README.md

ch07/README.md

@@ -299,7 +299,10 @@ Both are noting happened.
 
 ```cpp
 (a) Sales_data &combine(Sales_data); // ok
-(b) Sales_data &combine(Sales_data&); // [Error] no matching function for call to 'Sales_data::combine(std::string&)' (`std::string&` can not convert to `Sales_data` type.)
+(b) Sales_data &combine(Sales_data&); // [Error] no matching function for call to 'Sales_data::combine(std::string&)' (`std::string&` can not convert to `Sales_data` type.)  
+//It's wrong. Because combine’s parameter is  a non-const reference , we can't  pass a temporary to that parameter. If combine’s parameter is  a  reference to const , we can  pass a temporary to that parameter. Like this :Sales_data &combine(const Sales_data&);   Here we call the Sales_data combine member function with a string argument. This call is perfectly legal; the compiler
+automatically creates a Sales_data object from the given string. That newly generated (temporary) Sales_data is passed to
+combine.
 (c) Sales_data &combine(const Sales_data&) const; // [Error] assignment of member 'Sales_data::units_sold' in read-only object. (we cannot combine the other `Sales_data` object.)
 ```