Document graphics device setup

[mjskay]

Would be helpful to folks to have somewhere to point folks to help them configure a graphics device that supports blending. Maybe a mini vignette.

• [ ] a list of (currently) viable graphics devices
• [ ] how to configure them to support blending in common setups (script, rmarkdown, rstudio, ... maybe pkgdown?)
• [ ] pointer to Paul Murrell's article
• [ ] update the check_blend(), check_affine_transform() warning messages to point to the mini-vignette.
• [ ] update the README to point to the mini-vignette.