Update documentation.

README.md

@@ -375,6 +375,13 @@ and the function that performs this operation.
 Without an oplist, M\*LIB has no way to known how to deal with your type
 and will deal with it like a classic C type.
 
+When you define an instance of a new container, you give the oplist of the type
+you want to use as the base of the container. In function of the available interface
+of the oplist, the generation macro function generates the interface of the container.
+Then using the oplist of the container you can chain this new interface to another
+container:
+![oplist and definition](https://raw.githubusercontent.com/P-p-H-d/mlib/master/doc/oplist.png)
+
 A function name can be followed by the token M\_IPTR in the oplist
 (for example: (INIT(init\_func M\_IPTR)) )
 to specify that the function accept as its *first* argument a pointer