Update README.md (#2632)

Add example of specific spiffe format

feature/security/gnsi/certz/client_certificates/README.md

@@ -8,6 +8,20 @@ identification information. The client certificate should have a
 SPIFFE Idenitifier embedded in it to be used as the identifier of
 the client to the server.
 
+* SPIFFEE ID format
+
+```
+spiffe://<issuing role>.<context>.<security realm>.<domain>/role/<role>
+```
+
+* Example:
+
+```
+URI:spiffe://ca-issuer.sdn.wan.example.com/role/controller-role
+```
+
+
+
 ## Baseline Setup
 
 ### Input Args