title: circleCI - How to run job when has change in specific directory
body: <p>I'm using circleCI version 2 and my <code>config.yml</code> like this:</p>
  
  <pre><code>version: 2
  jobs:
     a:
       steps:...
     b:
       steps:...
  workflows:
    version: 2
    main_pipeline:
      jobs:
         - a
         - b
  </code></pre>
  
  <p>I want only to build when a change happens in the directory.</p>
  
  <p><code>job a</code> for folder a
  <code>job b</code> for folder b</p>
  
  <p>when <code>folder a</code> changes, build only <code>job a</code>.</p>