Update documentation for schema definition (dagu-org#645)

docs/source/yaml_format.rst

@@ -34,6 +34,18 @@ The command can be string or list of strings. The list of strings is useful when
 
 .. _specifying working dir:
 
+Schema Definition
+~~~~~~~~~~~~~~~~~~
+
+We have a schema definition for the DAG file. The schema definition is used to validate the DAG file. The schema definition is available at `dag.schema.json <https://github.com/daguflow/dagu/blob/main/schemas/dag.schema.json>`_. This schema can be used by IDEs to provide auto-completion and validation. I.e.,:
+
+.. code-block:: yaml
+
+  # yaml-language-server: $schema=https://raw.githubusercontent.com/daguflow/dagu/main/schemas/dag.schema.json
+  steps:
+    - name: step 1
+      command: echo hello
+
 Working Directory
 ~~~~~~~~~~~~~~~~~~