HYDRA-861 : Add note on what happens before each test

test/lib/mayaUsd/render/mayaToHydra/README.md

@@ -65,6 +65,9 @@ To add a new Python-only test suite :
     ```
 7. Add the name of your test[...].py file to the list of `TEST_SCRIPT_FILES` in the [current folder's CMakeLists.txt](./CMakeLists.txt)
 
+Some important notes :
+- Before each test, a new file will be opened and the renderer will be switched to Hydra. If you need to switch between renderers, you can use the `self.setHdStormRenderer()` and `self.setViewport2Renderer()` methods.
+
 # Best practices
 - Don't skip tests unless necessary. If a test requires a certain plugin to be loaded, don't skip the test if the plugin fails to load, as this will falsely be reported as a pass. For such cases, prefer setting the `_extraPluginsToLoad` variable in your test class.