Tutorial should mention Ember Observer

[jenweber]

In places like the deploying section of the Super Rentals tutorial, we should be mentioning looking at Ember Observer to find Ember Addons that may be helpful.

There may be multiple places that it should be mentioned, so please feel free to look through the Guides for any additional spots where it seems relevant to make this reference.

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Note, this issue is set aside for the EmberConf contributors workshop on 3/12. If it's not claimed by the end of that day, feel free to grab it!