Adds a simple example of a LabelPrintingMixin plugin

docs/extend/plugins/label.md

@@ -4,12 +4,49 @@ title: Label Mixin
 
 ## LabelPrintingMixin
 
-The `LabelPrintingMixin` class enables plugins to print labels directly to a connected printer.
+The `LabelPrintingMixin` class enables plugins to print labels directly to a connected printer. Custom plugins can be written to support any printer backend.
 
 An example of this is the [inventree-brother-plugin](https://github.com/inventree/inventree-brother-plugin) which provides native support for the Brother QL and PT series of networked label printers.
 
+### Web Integration
+
 If label printing plugins are enabled, they are able to be used directly from the InvenTree web interface:
 
 {% with id="label_print", url="plugin/print_label_select_plugin.png", description="Print label via plugin" %}
 {% include 'img.html' %}
-{% endwith %}
+{% endwith %}
+
+### App Integration
+
+Label printing plugins also allow direct printing of labels via the [mobile app](../../app/stock.md#print-label)
+
+## Implementation
+
+Plugins which implement the `LabelPrintingMixin` mixin class must provide a `print_label` function:
+
+```python
+from dummy_printer import printer_backend
+
+class MyLabelPrinter(LabelPrintingMixin, IntegrationPluginBase):
+    """
+    A simple example plugin which provides support for a dummy printer.
+
+    A more complex plugin would communicate with an actual printer!
+    """
+
+    PLUGIN_NAME = "MyLabelPrinter"
+    PLUGIN_SLUG = "mylabel"
+    PLUGIN_TITLE = "A dummy printer"
+
+    def print_label(self, label):
+        """
+        Send the label to the printer
+        
+        Arguments:
+            label: A PIL (pillow) Image file
+        """
+
+        printer_backend.print(label)
+```
+
+