add mermaid diagram to README

doc/README.md

@@ -8,4 +8,32 @@ binding according to the `free_interface` abstract interface defined in
 `sp_resource_m`.  The user-defined `free` subroutine must free the associated
 resource, which usually means deallocating the associated memory.
 
-![smart-pointers-class-diagram](https://user-images.githubusercontent.com/13108868/192410592-19c6e194-ac3e-4f5c-a308-33b2e10d69a7.png)
+```mermaid
+---
+title: Smart Pointers Class Diagram 
+---
+classDiagram
+
+class sp_resource_t {
+    free()
+}
+    <<Abstract>> sp_resource_t
+    sp_resource_t--*sp_reference_counter_t
+
+class sp_reference_counter_t {
+    -count_: integer, pointer
+    -object_: sp_resource_t
+    grab()
+    release()   
+}
+sp_reference_counter_t--*sp_smart_pointer_t
+
+class sp_smart_pointer_t {
+    -counter_:counter_t
+    release_handle()
+    start_counter()
+}
+<<Abstract>> sp_smart_pointer_t
+    sp_smart_pointer_t--|>sp_resource_t
+
+```