[BioC] (SUMMARY) warning if pdf of vignette is missing

[Seth Falcon]

For the "final" step of checking your packages, we recommend the
following:

R CMD build foo/
R CMD check foo_1.2.3.tar.gz

This procedure has the following advantages:

1. Vignette pdfs are generated by the build step and checked in the
   check step.

2. Running check in this way will not have any side-effects (left
   behind files) in your package source directory.

It is, of course, a bit tedious to build and check.  So while you are
working on code, you may find it convenient to occasionally run check
on the package source directory.  

Hope that helps clarify things.

+ seth