Add comment to generated template package

[krlmlr]

for completeness.

Add-on to #129.

[kevinushey]

I think that, rather than adding this as a comment in src/Makevars, we should either:

1. Write it to the NAMESPACE directly,
2. Write a comment out to the user (via message()) saying "Please add importFrom(RcppParallel, ...) to ensure RcppParallel is loaded when your package is used."

[krlmlr]

This comment would have spared ~2 hours of research if it had been there.

The directive is already written to NAMESPACE. It is lost the first time the user calls devtools::document(). It's much more difficult to lose that comment from Makevars, this file isn't touched by automated tools AFAICT.