Add report mixin page (#473)

* Add report mixin page

* Add example

* Add link from report page

docs/extend/plugins/panel.md

@@ -157,6 +157,5 @@ Here are some examples of available colors:
 
 Please have a look at the css files for more options. The last line renders the value that was defined in the plugin. 
 
-Just give it a try: Each time you press the button, the value will be increased.
-
-Have fun
+!!! tip "Give it a try"
+    Each time you press the button, the value will be increased.

docs/extend/plugins/report.md

@@ -0,0 +1,52 @@
+---
+title: Report Mixin
+---
+
+## ReportMixin
+
+The ReportMixin class provides a plugin with the ability to extend the functionality of custom [report templates](../../report/report.md). A plugin which implements the ReportMixin mixin class can add custom context data to a report template for rendering.
+
+### Example
+
+A sample plugin which provides additional context data to the report templates can be found [in the InvenTree source code](https://github.com/inventree/InvenTree/blob/master/InvenTree/plugin/samples/integration/report_plugin_sample.py):
+
+```python
+"""Sample plugin for extending reporting functionality"""
+
+import random
+
+from plugin import InvenTreePlugin
+from plugin.mixins import ReportMixin
+from report.models import PurchaseOrderReport
+
+
+class SampleReportPlugin(ReportMixin, InvenTreePlugin):
+    """Sample plugin which provides extra context data to a report"""
+
+    NAME = "Report Plugin"
+    SLUG = "reportexample"
+    TITLE = "Sample Report Plugin"
+    DESCRIPTION = "A sample plugin which provides extra context data to a report"
+    VERSION = "1.0"
+
+    def some_custom_function(self):
+        """Some custom function which is not required for the plugin to function"""
+        return random.randint(0, 100)
+
+    def add_report_context(self, instance, request, context):
+        """Add example content to the report instance"""
+
+        # We can add any extra context data we want to the report
+
+        # Generate a random string of data
+        context['random_text'] = ''.join(random.choices('abcdefghijklmnopqrstuvwxyz', k=20))
+
+        # Call a custom method
+        context['random_int'] = self.some_custom_function()
+
+        # We can also add extra data to the context which is specific to the report type
+        context['is_purchase_order'] = isinstance(instance, PurchaseOrderReport)
+
+        # We can also use the 'request' object to add extra context data
+        context['request_method'] = request.method
+```