remove instructions on R CMD build, about which probably nobody cares
move the contributing section to CONTRIBUTING.md
add @tibastral for #900
tweak contributing.md
fixes #898: as.data.frame(x) does not work when x is a matrix with duplicate row names
closes #897: the path to the `convert` utility of ImageMagick can be specified via the chunk option `engine.opts`
closes #896: allow custom command line arguments to ImageMagick/convert
fixes #893 introdcued by 7bb9623e: I do not load rmarkdown at package build time now (see 6abcb5320f6), so it is okay to call rstudio::pandoc_available() inside my pandoc_available() function now
tweak contributing.md again
bump version