Expand libraries vignette with PACKAGE_types.h

[eddelbuettel]

This comes from a discussion over at RcppEigen which revealed that our trick of using PACKAGE_types.h to declare types known to RcppExports.cpp is not as well known.

So a brief reference was added to Rcpp-libraries vignette which is somewhat topical.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[Enchufa2]

Typo below; otherwise, LGTM!