Update README

Document ENV vars with examples.
Update README.markdown
Merge pull request #220 from casharma/update-readme

Update README
Do NOT remove the PE installer

- The Advanced kickstand lab requires the PE installer tarball.
- This relates to TRAINVM-43
Merge pull request #221 from casharma/restore/installer

Do NOT remove the PE installer.

Sorry :(
don't delete the pe installer after extracting, on training VMs
on the learning VM, delete the installer after extraction
Merge pull request #222 from casharma/fix/extraction

Fix/extraction
Merge pull request #223 from binford2k/master

Added the fixes to get the classroom running with advanced
Added a simple external node script for debugging

This just duplicates the functionality of the old external_node script.
This is mainly a bandaid to keep the current course afloat while we
refactor.
added a comment explaining this
also, quote--just for carefuls
fix the quoting
Merge pull request #225 from binford2k/fix/3.2/external_node_script

Added a simple external node script for debugging