[None][doc] update readme for rpc (#9972)

Signed-off-by: Yan Chunwei <328693+Superjomn@users.noreply.github.com>

Author: Superjomn

tensorrt_llm/executor/rpc/README.md

@@ -83,3 +83,8 @@ except RPCError as e:
 # Shutdown server from client
 client.shutdown_server()
 ```
+
+## Network Security
+
+The RPC supports built-in HMAC-based authentication to secure the communication between the server and the client. 
+To enable that, you need to provide a shared secret key (bytes) to both the `RPCServer` and `RPCClient`.