added model link documentation

readme.md

@@ -62,6 +62,18 @@ def generate_key(self):
     self.save()
 ```
 
-To make a class operation instead of one applicable to a specific instance then add the ``@classmethod`` decorator to the function.
+To make a class operation instead of one applicable only to a specific instance then add the ``@classmethod`` decorator to the function.
+
+## Model Links
+
+A link can be added to a model to provide a direct access from the admin interface to a location of interest related to the model. It is defined by decorating a function that returns an URL with the ``@appier.link`` decorator. This decorator accepts a name describing the link and a tuple of parameters like in the ``@appier.operation`` decorator. By adding the ``@classmethod`` decorator the link will be related to the model class:
+
+```python
+@classmethod
+@appier.operation(name = "Export CSV")
+def export_csv_url(cls):
+    return appier.get_app().url_for("list.csv")
+```
+