title: how to include a standalone file in the built bookdown website?
body: <p>i am trying to include a custom <code>404.html</code> file in the bookdown <code>/docs/</code> output folder of my <code>:gitbook:</code> output.  if i just copy the file into the <code>/docs/</code> then it ends up getting deleted any time i run <code>clean_site()</code> so i'm not sure if i'm supposed to be placing the file elsewhere?</p>
  
  <p><a href="http://rmarkdown.rstudio.com/rmarkdown_websites.html#site_configuration" rel="nofollow noreferrer">this <code>rmarkdown</code> page</a> makes it look like i should just be able to include the file in the project's home directory <code>/</code> but that doesn't appear to work with <code>render_site()</code>.</p>
  
  <p><a href="https://bookdown.org/yihui/bookdown/yaml-options.html" rel="nofollow noreferrer">this bookdown.org page</a> explains how to add a snippet of html code to the html pages rendered, but not a standalone file.</p>
  
  <p>sorry if i'm missing this somewhere obvious.  thanks!</p>