title: Number some sections with rmarkdown
body: <p>I am writing a manuscript with rmarkdown.</p>
  
  <p>If I want to number all sections, I can use YAML like this </p>
  
  <pre><code>---
  title: "My Report"
  output: 
    html_document:
      number_sections: true
  ---
  </code></pre>
  
  <p>See <a href="https://stackoverflow.com/questions/40120050/automatically-number-sections-in-rmarkdown/40157952">Automatically number sections in RMarkdown</a></p>
  
  <p>But I only want to number some sections, so my document looks like</p>
  
  <blockquote>
    <p><strong>Abstract</strong><br>
     <strong>1. Introduction</strong><br>
     <strong>2. Methods</strong><br>
     <strong>3. Results</strong><br>
     <strong>References</strong>  </p>
  </blockquote>
  
  <p>Is there anyway to do this?</p>