Fix: 06.shipments.md (#76)

* Update: 06.shipments.md

Add ZPL label info

* Update 06.shipments.md

* Update 06.shipments.md

src/api-reference/06.shipments.md

@@ -1593,6 +1593,14 @@ labels, this process will take between 3 seconds and 3 minutes. You can
 regularly check whether the label is available or you can use
 the <ApiLink to="11_C">label_created webhooks</ApiLink>.
 
+When you want to retrieve ZPL labels you need to use a different Accept header.
+'Accept: application/vnd.shipment_label+zpl' or 'Accept: application/vnd.zpl' You will now receive the ZPL label.
+Currently, our system supports only PostNL labels in ZPL format for the following shipment types:
+- Domestic shipments for both normal packages and mailbox packages.
+- EU shipments
+If you request a rest of world label, the system will return a label in PDF format.
+Additionally, it's important to note that you can request just one label at a time.
+
 ### 6.F.2 Reference
 
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