title: knitr .Rmd vignettes do not appear with vignette()
body: <p>In a package I'm developing with R Studio, I create vignettes via <code>devtools::use_vignette("mydoc.Rnw")</code>, which gives a standard vignette header like</p>
  
  <pre><code>---
  title: "Title"
  author: "Michael Friendly"
  date: "`r Sys.Date()`"
  output: rmarkdown::html_vignette
  vignette: &gt;
    %\VignetteIndexEntry{Title}
    %\VignetteEngine{knitr::rmarkdown}
    %\VignetteEncoding{UTF-8}
  ---
  </code></pre>
  
  <p>I have followed all the instructions in <a href="http://yihui.name/knitr/demo/vignette/">http://yihui.name/knitr/demo/vignette/</a> and <a href="http://r-pkgs.had.co.nz/vignettes.html">http://r-pkgs.had.co.nz/vignettes.html</a>.  The vignettes <strong>are</strong> listed on the CRAN page for the package, yet they seem inaccessible in an R session with the package loaded.</p>
  
  <pre><code> &gt; browseVignettes("matlib")
   No vignettes found by browseVignettes("matlib") 
  
  &gt; library(tools)
  &gt; names(vignetteEngine(package = 'matlib'))
  Error in getEngine(name, package) : 
    None of packages ‘matlib’ have registered vignette engines
  </code></pre>
  
  <p>I know that other packages with <code>knitr</code>-processed .Rmd vignettes are accessible from the package, but can't figure out why mine are not.
  What is missing?  </p>
  
  <p>My <code>vignettes/</code> directory contains only the .Rmd files (no PDFs), but that seems the same as, e.g., <a href="https://github.com/yihui/knitr/tree/master/vignettes">https://github.com/yihui/knitr/tree/master/vignettes</a>.</p>