Docs from PS:
http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/#50577409_pgfId-1037504

To load them one needs to:

1. Process block 0x42D and get IDs of the selected layers
2. Process 'lyid' additional section of the layers
3. Activate layers with KisDocument::setPreActivatedNode()

To save them:

1. Get layers list with KisDocument::activeNodes()
2. Assign 'lyid' sections to all the layers
3. Save resource block 0x42D