Add vignette skeleton to document randomization ratio behaviors

[nanxstats]

Randomization ratio seems to be an important and complex enough input (or inferred) parameter that warrants somewhat detailed documentation.

I will create a vignette skeleton to get things started, with the diagram.

[LittleBeannie]

Hi @nanxstats, do you have a timeline to get this finished?

[nanxstats]

Nope, apparently this has not been able to make my priorities list so I will close this as won't do.

In case someone is interested, some relevant information in #281.