Add hello world to tutorial

[jtremback]

Close #20

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[izqui]

Thanks for your contribution, this will definitely help! Just a couple of details, and as soon as the CLA is accepted by you, we can merge.

[izqui]

As explained aragon/aragon-cli#148 (comment), npm install shouldn't be required as it is already performed by the init command.

[izqui]

I would substitute npm start with aragon run to be consistent with the rest of the documentation.

[luisivan]

Applied all changes by @izqui