CmdStan User's Guide - document "Aki trick" to see initial parameter values

[mitzimorris]

Summary:

https://discourse.mc-stan.org/t/does-the-init-argument-work-with-cmdstanr/17255/16

Description:

per discussion on discourse - by running the sampler with fixed params and
inspecting the Stan csv output file, you can see all of the parameter values
reported on the constrained scale which corresponds to how they are specified
by the user.

Additional Information:

Provide any additional information here.

Current Version:

v2.18.0

[jgabry]

Hopefully eventually this can be removed and we can actually provide access to initial values, but yeah it would be great to document this until then. I just opened an issue at stan-dev/stan: stan-dev/stan#2950

[mitzimorris]

#261