title: node/no-unpublished-import: "chai" is not published
body: <p>I have a <a href="https://www.npmjs.com/package/chai" rel="noreferrer">chai</a> among my <code>devDependencies</code>:</p>
  <pre class="lang-json prettyprint-override"><code>&quot;devDependencies&quot;: {
      &quot;@types/chai&quot;: &quot;4.2.14&quot;,
      &quot;chai&quot;: &quot;4.2.0&quot;,
  }
  </code></pre>
  <p>Chai is requires for tests only, but I don't going to publish the test files.
  The rule <a href="https://github.com/mysticatea/eslint-plugin-node/blob/master/docs/rules/no-unpublished-import.md" rel="noreferrer">node/no-unpublished-import</a> tells me:</p>
  <pre><code>&quot;chai&quot; is not published 
  </code></pre>
  <p>From the documentation:</p>
  <blockquote>
  <p>This intends to prevent &quot;Module Not Found&quot; error after npm publish. 
  If you want to import devDependencies, please write .npmignore or
  &quot;files&quot; field of package.json.</p>
  </blockquote>
  <p>I did not understand what means &quot;write .npmignore&quot; advice. What I need to write there?
  Currently, I have below content:</p>
  <pre><code>.idea/
  
  /.eslintignore
  /.eslintrc.yaml
  /.gitignore
  /.mocharc.yaml
  /.npmignore
  /tsconfig.json
  
  **.test.ts
  </code></pre>
  <p>The last one ignores all test files using chai.</p>