find_external_resources(input_file, encoding = getOption("encoding"))  In R Markdown documents, it looks for references to files implicitly
  referenced in Markdown (e.g. ), in the document's
  YAML header, in raw HTML chunks, and as quoted strings in R code chunks
  (e.g. read.csv("data.csv")).
  Resources specified explicitly in the YAML header for R Markdown documents
  are also returned. To specify resources in YAML, use the
  resource_files key:
--- title: My Document author: My Name resource_files: - data/mydata.csv - images/figure.png ---
  Each item in the resource_files list can refer to:
  
images/figure.png, or
  resources/data, in which case all of the
    directory's content will be recursively included, or
  data/*.csv, in which case all of
    the files matching the pattern will be included. No recursion is done in
    this case.
  
  In HTML files (and raw HTML chunks in R Markdown documents), this routine
  searches for resources specified in common tag attributes, such as
  , etc.
In all cases, only resources that exist on disk and are contained in the document's directory (or a child thereof) are returned.