Update README.md

Add description to node "optional"

README.md

@@ -7,6 +7,7 @@ flowchart TD
         switch2("Switch 2")
         switch1("Switch 1")
     subgraph OPTIONAL
+        explain@{shape: text, label: "If this section is omitted, connect **Switch 2** directly to **Server**"}
         switch3("Switch 3")
         tls("TLS Appliance")
     end
@@ -15,7 +16,6 @@ flowchart TD
         client("Client Computer")
     collector("Collector Computer") o-- Mirror --o switch1 & switch2 & switch3   
   end
-
   subgraph Sharam
         computer1["First computer"] o--o sswitch1[Switch 1] o--o dut["device under test"] o--o sswitch2[Switch 2] o--o computer2["Second Computer"] 
         switch3("Switch 3")