Adding child nodes to existing nodes in XML using Powershell

I have the following XML structure:

<ovf:Envelope>
  <References>
  </References>
  <DiskSection>
  </DiskSection>
  <NetworkSection>
  </NetworkSection>
<vmw:IpAssignmentSection>
    <Info>Supported IP assignment schemes</Info>
</vmw:IpAssignmentSection>
</ovf:Envelope>  

I want to add another node <Info> as a child node to <DiskSection> with Text as "Disk Section".

Can someone please help me with the correct code sequence in powershell to achieve that? Urgent!