Admit defeat, for now, at getting high res PNG

In 03_doStuff.Rmd I embed this PNG and it is erroneously interpreted as having 72ppi. And given the very high number of pixels, this has undesirable effect of image being truly huge when embedded in HTML.
Add an .Rmd file to a hw06 example
Finish: Add an .Rmd file to a hw06 example
Fix many links and mention new .Rmd file