update usage

README.md

@@ -32,7 +32,8 @@ namespace mgutility{
     namespace detail{
         // specialize rgb_color::unknown to make output "UNKNOWN"
         template <>
-        constexpr inline auto enum_type::name<rgb_color, rgb_color::unknown>() noexcept -> string_view { return "UNKNOWN"; }
+        constexpr inline auto enum_type::name<rgb_color, rgb_color::unknown>() noexcept 
+                                                    -> string_view { return "UNKNOWN"; }
     }
 
     template<>
@@ -51,6 +52,12 @@ int main()
 {
     auto x = rgb_color::blue;
     auto y = mgutility::to_enum<rgb_color>("greenn");
+
+    for (auto&& e : mgutility::enum_for_each<rgb_color>()) {
+        std::cout << e.name << " " << e.to_underlying() << '\n'; 
+        // enum_pair<Enum> has two data members: name and value 
+        // to_underlying() converts into underlying type of enum
+    }
 
     // default signature: enum_name<min_value = -128, max_value = 128, Enum typename>(Enum&&) 
     // Changing max_value to not too much greater than enum's max value, it will compiles faster