message: add documentation for verbose logging
diff: @@ -44,7 +44,15 @@ bazel test --config macos_x86_64 @com_intel_plaidml//plaidml/keras:backend_test
  ## Enable Verbose Logging
  -If you are importing the `plaidml` package in your code, you can enable verbose
  -logging with the function `plaidml_set_internal_vlog`. The function takes a
  -single integer argument specifying the level of verbosity (available levels are
  -1-4 inclusive).
  +You can enable verbose logging through the environment variable
  +`PLAIDML_VERBOSE`.
  +
  +`PLAIDML_VERBOSE` should be set to an integer specifying the level of verbosity
  +(valid levels are 0-4 inclusive, where 0 is not verbose and 4 is the most
  +verbose).
  +
  +For instance, the following command would set a verbosity level of 1.
  +
  +```
  +export PLAIDML_VERBOSE=1
  +```
  