Brief documentation on the template tag

Author: Mark Gibbs

docs/template_tags.rst

@@ -3,3 +3,17 @@
 Template tags
 =============
 
+The ``plotly_dash`` library provides the ``plotly_app`` template tag. This tag inserts
+a ``Dash`` app within a page as a responsive ``iframe`` element.
+
+:name = None: The name of the application, as passed to a ``DjangoDash`` constructor.
+:slug = None: The slug of the application instance.
+:da = None: An existing ``django_plotly_dash.models.DashApp`` model instance.
+:ratio = 0.1: The ratio of height to width. The container will inherit its width as 100% of its parent, and then rely on
+              this ratio to set its height.
+:use_frameborder = "0": HTML element property of the iframe containing the application.
+
+At least one of ``da``, ``slug`` and ``name`` must be provided. An object identified by ``slug`` will always be used, otherwise any
+identified by ``name`` will be. If either of these arguments are provided, they must resolve to valid objects even if
+not used. If neither are provided, then the model instance in ``da`` will be used.
+