The build_readme() function (build-readme.R) finds the file to build using the following regular expression:
regexp <- paste0(path_file(pkg$path), "/(inst/)?readme[.]rmd")
This will match any filename that includes at least readme.rmd somewhere in the name. This includes emacs backup files (readme.rmd~) among possibly others; undoubtedly such extraneous files should be ignored. An exact match to readme.rmd can be accomplished by changing the regexp (see attached patch).
build-readme.R.patch