Add new vignette about packaging C++ libraries (closes #1077)

[eddelbuettel]

As discussed in #1077, we have a new vignette candidate. And there seems to be some support for including it.

Checklist

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

[codecov-io]

Codecov Report

Merging #1078 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1078   +/-   ##
=======================================
  Coverage   95.55%   95.55%           
=======================================
  Files          64       64           
  Lines        2787     2787           
=======================================
  Hits         2663     2663           
  Misses        124      124           

[eddelbuettel]

Will go slightly off usual schedule and merge this -- as it leaves inst/include, src/, R/ and inst/tinytest alone there should really be no surprise from this on the behavior side.