Make clear to use Session BEFORE app->boot
minor #949 Make clear to use Session BEFORE app->boot (sebastian-marinescu)

This PR was squashed before being merged into the 2.0.x-dev branch (closes #949).

Discussion
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Make clear to use Session BEFORE app->boot

It wasn't clear to me the session needs to be there before you boot.
I 'wasted' a great amount of time finding that out.
Please clarify for upcoming geniuses like me.

Commits
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5ddcc8b Make clear to use Session BEFORE app->boot