Add documentation section for socat PTYs

README.md

@@ -68,4 +68,14 @@ socat -d -d -v pty,rawer,link=$(pwd)/testPty EXEC:./clientsim,pty,rawer
 ```
 
 You will need to configure `$(pwd)/testPty` as `serial`/`device` in your `config.yaml` to let the signer command find
-the emulated serial device.
+the emulated serial device.
+
+## Using socat to simulate TTYs for signer and signer client
+
+The following `socat` command line will open TTY pseudo devices in `./testPty` as well as
+`../cacert-gosignerclient/testPty`. This will allow both the signer and the signer client code to have
+`testPty` as `device` value in their configuration YAML file.
+
+```shell
+socat -d -d pty,rawer,link=$(pwd)/testPty pty,rawer,link=$(pwd)/../cacert-gosignerclient/testPty
+```