Added python 2.7 note in hello_world.
Added python 2.7 note in hello_world.

BadWizard/intro_programming,nntisapeh/intro_programming,chenleo/ipynotebook,nntisapeh/intro_programming,ehmatthes/intro_programming,chenleo/ipynotebook,nntisapeh/intro_programming,BadWizard/intro_programming,ehmatthes/intro_programming,BadWizard/intro_programming,BadWizard/intro_programming,ehmatthes/intro_programming,ehmatthes/intro_programming,nntisapeh/intro_programming,chenleo/ipynotebook,chenleo/ipynotebook