Documentation for setting up development environment should be updated with system dependencies

[ipwnponies]

These are issues we experienced recently while setting up development environment:

• Minimum node version is required, works for 7.2 (6.1.0 does not have --inspect argument).
• After google app engine is installed and symlinks created, $PYTHONPATH needs to be exported

Add development tip to run make serve and npm run webpack -- --watch in background. This will run webserver and also have webpack watch directory for changes and recompile as necessary.