Add SVG diagram example

FYI, I've used the amazing Excalidraw service to create this diagram

Author: pH-7

example-web-app-REST-API-architecture.svg

@@ -1,7 +1,9 @@
-# Build aPHP RESTful API 
+# Build a PHP RESTful API
 
 This is the source code of my Udemy course **Build a Modern REST API with PHP 8**
 
+![Diagram showing example of REST API architecture](example-web-app-REST-API-architecture.svg)
+
 
 ## The course