Added a chapter for more complicated data (#5878)

docs/docs/extend/plugins/panel.md

@@ -254,3 +254,61 @@ that does the POST request, handles errors and the csrftoken.
 !!! tip "Give it a try"
     change the values in the fields and push Save. You will see the values
     in the InvenTree log.
+
+#### If things are getting more complicated
+
+In the example above we code all parameters into the URL. This is easy and OK
+if you transfer just a few values. But the method has at least two disadvantages:
+
+* When you have more parameters, things will get messy.
+* When you have free text input fields, the user might enter characters that are not allowed in URL.
+
+For those cases it is better to pack the data into a json container and transfer
+this in the body of the request message. The changes are simple. Lets start with
+the javascript:
+
+```html
+{% raw %}
+<script>
+async function example_select(){
+    const layernumber = parseInt(document.getElementById("layer_number").value)
+    const size = document.getElementById("string").value
+    const cmd_url="{% url 'plugin:examplepanel:transfer' %}";
+    data = {
+        layer_number: layer_number,
+        size: size
+    }
+    response = inventreeFormDataUpload(url=cmd_url, data=JSON.stringify(data))
+}
+</script>
+{% endraw %}
+```
+
+Here we create a json container (data). The function stringify converts this to the
+proper string format for transfer. That's all. The function inventreeFormDataUpload
+does the rest of the work.
+
+The python code in the plugin also needs minor changes:
+
+```python
+from django.conf.urls import url
+import json
+
+...
+
+    def setup_urls(self):
+        return [
+                url(r'example(?:\.(?P<format>json))?$', self.do_something, name='transfer'),
+        ]
+
+# Define the function that will be called.
+    def do_something(self, request):
+
+        data=json.loads(request.body)
+        print('Data received:', data)
+```
+
+The URL and the called function have no parameter names any longer. All data is in the
+request message and can be extracted from this using json.loads. If more data is needed
+just add it to the json container. No further changes are needed. It's really simple :-)
+