add custom directory location note

Author: jonkrohn

installation/step_by_step_MacOSX_install.md

@@ -12,7 +12,7 @@
 6. Back in Terminal, execute `source TensorFlow-LiveLessons/installation/let_jovyan_write.sh` so that you can write to files in the *TensorFlow-LiveLessons* directory from inside the Docker container we'll be creating momentarily 
 7. Move into the *TensorFlow-LiveLessons* directory by executing `cd TensorFlow-LiveLessons`
 8. Build the Docker container by executing `sudo docker build -t tensorflow-ll-stack .` (you'll get an error if you miss the final `.`!)
-9. When that build process has finished, run the Docker container by executing `sudo docker run -v ~/TensorFlow-LiveLessons:/home/jovyan/work -it --rm -p 8888:8888 tensorflow-ll-stack`
+9. When that build process has finished, run the Docker container by executing `sudo docker run -v ~/TensorFlow-LiveLessons:/home/jovyan/work -it --rm -p 8888:8888 tensorflow-ll-stack` (if you modified steps two or three above, e.g., by running `git clone` somewhere other than your home `~` directory, then you will need to similarly modify the `~/TensorFlow-LiveLessons` directory location portion of this command)
 10. In the web browser of your choice (e.g., Chrome), copy and paste the URL created by Docker (this begins with `http://localhost:8888/?token=` and should be visible near the bottom of your Terminal window) 
 
 ## Shutdown