Cleanups to MacOS setup section in README.md.
Cleanups to MacOS setup section in README.md.

The directions now read more clearly and also prevent a possible error
by making sure a configuration file exists before running
config/setup_node.py.

jimklo/LearningRegistry,jimklo/LearningRegistry,LearningRegistry/LearningRegistry,jimklo/LearningRegistry,LearningRegistry/LearningRegistry,jimklo/LearningRegistry,jimklo/LearningRegistry,jimklo/LearningRegistry,LearningRegistry/LearningRegistry,LearningRegistry/LearningRegistry,jimklo/LearningRegistry,LearningRegistry/LearningRegistry,LearningRegistry/LearningRegistry,jimklo/LearningRegistry,LearningRegistry/LearningRegistry,jimklo/LearningRegistry