Add version warning to throughput test

Author: vishalbollu

examples/utils/throughput_test.py

@@ -1,3 +1,5 @@
+# WARNING: you are on the master branch; please refer to examples on the branch corresponding to your `cortex version` (e.g. for version 0.19.*, run `git checkout -b 0.19` or switch to the `0.19` branch on GitHub)
+
 import os
 import sys
 import click