Document that `arc init`/`create` is mean to be rerun

[buffpojken]

Is your feature request related to a problem? Please describe.
It's not obvious, both from the docs structure and the naming that arc init is meant to be rerun in order to generate new code for changes in the arc-file. I could only find due to expecting it somewhere from past experience.

Describe the solution you'd like
A section added to Quickstart which walks through adding a function, and then adding another one - running arc init as part of the process. This is the first place a new user looks, and it makes it obvious that any changes to the arc-file requires a re-run of init.

Describe alternatives you've considered
A more complete guide to make a trivial app from start to finish. I'd be willing to begin work on this if this is something you'd like to include.

[filmaj]

This is a great suggestion! I would like to collaborate on that effort in a PR if you're willing to open one?

[kristoferjoseph]

Great addition.
This behavior just changed recently so it would be great to show how to use it by default as well as configure autocreate to get the previous behavior if you like.

Set autocreate to true for previous default behavior.