Update documentation for Python tests

README.developer

@@ -481,6 +481,21 @@ autogenerated tests must be updated:
 
 and the handwritten tests should be updated to reflect the change.
 
+The Python tests can also be run against a simulator or an actual rig, but
+they aren't guaranteed to succeed because the CI only tests the dummy rig.
+To execute the tests from the build tree, add the path to the libraries
+that you built, using the PYTHONPATH environment variable, eg:
+    PYTHONPATH=bindings/:bindings/.libs/ bindings/python/test_rig.py \
+    --model 1035 --rig-file /dev/ttyUSB0 --serial-speed 4800
+
+Only the following long arguments are supported:
+    --model ID
+    --rig-file DEVICE
+    --serial-speed BAUD
+    --hamlib-verbose
+The argument --hamlib-verbose can be repeated as many times as the --verbose
+argument accepted by rigctl.
+
 
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