Document the filtering support of DirectoryTree

docs/widgets/directory_tree.md

@@ -14,6 +14,26 @@ The example below creates a simple tree to navigate the current working director
 --8<-- "docs/examples/widgets/directory_tree.py"
 ```
 
+## Filtering
+
+There may be times where you want to filter what appears in the
+`DirectoryTree`. To do this inherit from `DirectoryTree` and implement your
+own version of the `filter_paths` method. It should take an iterable of
+Python `Path` objects, and return those that pass the filter. For example,
+if you wanted to take the above code an filter out all of the "hidden" files
+and directories:
+
+=== "Output"
+
+    ```{.textual path="docs/examples/widgets/directory_tree_filtered.py"}
+    ```
+
+=== "directory_tree_filtered.py"
+
+    ~~~python
+    --8<-- "docs/examples/widgets/directory_tree_filtered.py"
+    ~~~
+
 ## Messages
 
 ### ::: textual.widgets.DirectoryTree.FileSelected