title: Append a string after a multiple line match in bash
body: <p>I would need a <code>sed</code> or <code>awk</code> command to print a line after a multiple line match in bash. For example having this file as input:</p>
  
  <pre><code>admin:
    users:
      - user1
      - user2
  
  read_only:
    users:
      - user3
  
  write_only:
    users:
      - user4
  
  access_only:
    users:
      - user5
      - user6
  </code></pre>
  
  <p>..I would need to have this as output:</p>
  
  <pre><code>admin:
    users:
      NEW USER
      - user1
      - user2
  
  read_only:
    users:
      - user3
  
  write_only:
    users:
      - user4
  
  access_only:
    users:
      - user5
      - user6
  </code></pre>
  
  <p>The regex should match the first two lines <code>admin: users:</code> and then append <code>NEW USER</code> line.</p>
  
  <p>How to do this? Should I use sed or awk? How?
  Thank you</p>