step-by-step ss3 model guide

[k-doering-NOAA]

A while back, I started putting together a step-by-step guide to the details of an SS model. I thought this would be a useful resource (to compliment getting started and building your first SS model), as it walks through the SS input files and explains the basic components for a "simple" model (a simplified version of Simple). I thought this would be a useful resource for folks getting started with SS, but who are already familiar with other stock assessment models.

I think it makes sense to move the draft guide to this repo; any thoughts on whether or not this would be a helpful resource to keep developing?

[k-doering-NOAA]

A draft version is available in the repo: https://github.com/nmfs-stock-synthesis/doc/blob/8564c561d1d1265c99b7f31d7b4d2452b5f8685e/User_Guides/model_step_by_step/model_tutorial.Rmd

This could be removed if it doesn't seem worth developing further, but for now I will leave this issue open in case someone wants to improve it in the future.