add mermaid class diagram to README

Author: kareem-weaver

doc/README.md

@@ -8,4 +8,25 @@ 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
+classDiagram
+sp_resource_t --* sp_reference_counter_t
+sp_resource_t : free()
+
+sp_smart_pointer_t --|> sp_resource_t
+
+class sp_smart_pointer_t {
+-counter_ : counter_t
+release_handle()
+start_counter()
+}
+
+sp_reference_counter_t --* sp_smart_pointer_t 
+
+class sp_reference_counter_t {
+-count_ : integer, pointer
+-object_ : sp_resource_t
+grab()
+release()   
+}
+```