Updated v2_0_cpp_adaptor (markdown)

v2_0_cpp_adaptor.md

@@ -141,6 +141,24 @@ I use JSON to help understanding. But actual condition is msgpack format.
 
 When you use an array based adaptor, e.g.)MSGPACK_DEFINE_ARRAY, the order of member variables and base classes is important. The index of the msgpack::object array is corresponding to the member variables order. When you use a map based adaptor, e.g.)MSGPACK_DEFINE_MAP, the names of member variables and base classes is important. The keys of the msgpack::object map is corresponding to member variables names and the base classes names.
 
+##### since 2.1.0
+You can also customize the name of the map key using MSGPACK_NVP.
+For example, the key of the value `b` is `#b`. You can use any strings even if it can't use as C++ variable name. It helps inter programming language data transportation.
+
+```cpp
+#include <msgpack.hpp>
+
+struct your_class {
+    int a;
+    std::string b;
+    MSGPACK_DEFINE_MAP(a, MSGPACK_NVP("#b", b));
+};
+```
+
+```Javascript
+{"a":42,"#b":"ABC"}
+```
+
 #### non-intrusive approach
 When you don't want to modify your class, you can use non-intrusive approach. msgpack-c provides the following four functor class templates. Those templates are in the namespace msgpack::adaptor.