add docs for plaintext metadata format
add docs for plaintext metadata format

juliandunn/opscode-omnitruck,rhass/opscode-omnitruck,juliandunn/opscode-omnitruck,chef/opscode-omnitruck,glennmatthews/opscode-omnitruck,chef/omnitruck,chef/opscode-omnitruck,glennmatthews/opscode-omnitruck,glennmatthews/opscode-omnitruck,juliandunn/opscode-omnitruck,rhass/opscode-omnitruck,chef/opscode-omnitruck,chef/omnitruck,rhass/opscode-omnitruck,chef/omnitruck