add Stencila to user interface docs

docs/source/howto/user_interface.rst

@@ -31,10 +31,29 @@ To learn more about URLs in JupyterLab and Jupyter Notebook, visit
 RStudio
 -------
 
-The RStudio user interface is automatically enabled a configuration file for
-R is detected (an R version specified in ``runtime.txt``). If this is detected,
+The RStudio user interface is automatically enabled if a configuration file for
+R is detected (i.e. an R version specified in ``runtime.txt``). If this is detected,
 RStudio will be accessible by appending ``/rstudio`` to the URL, like so:
 
 .. code-block:: none
 
    http(s)://<server:port>/rstudio
+
+Stencila
+--------
+
+The Stencila user interface is automatically enabled if a Stencila document (i.e. 
+a file `manifest.xml`) is detected. Stencila will be accessible by appending 
+``/stencila`` to the URL, like so:
+
+.. code-block:: none
+
+   http(s)://<server:port>/stencila
+
+The editor will open the Stencila document corresponding to the last `manifest.xml`
+found in the file tree. If you want to open a different document, you can configure
+the path in the URL parameter `archive`:
+
+.. code-block:: none
+
+   http(s)://<server:port>/stencila/?archive=other-dir