Add a "real" example to Writing good messages section

[MichaelChirico]

One point not covered here explicitly -- avoid templating out sentence fragments. Almost a converse of the Un-translatable content section -- Put message pieces into a template if and only if they are untranslateable.

I can dig up examples from data.table going through this if that's not clear.

Originally posted by @MichaelChirico in #237 (comment)

Follow-up to #237