Add screencast

Author: SylvainCorlay

README.md

@@ -1,10 +1,8 @@
-
-# sidecar
+# Sidecar
 
 [![Build Status](https://travis-ci.org/jupyter-widgets/jupyterlab-sidecar.svg?branch=master)](https://travis-ci.org/jupyter-widgets/jupyterlab-sidecar)
 [![codecov](https://codecov.io/gh/jupyter-widgets/jupyterlab-sidecar/branch/master/graph/badge.svg)](https://codecov.io/gh/jupyter-widgets/jupyterlab-sidecar)
 
-
 A sidecar output widget for JupyterLab
 
 ## Installation
@@ -18,6 +16,9 @@ jupyter labextension install @jupyter-widgets/jupyterlab-sidecar
 
 ## Usage
 
+The sidecar widget is used as a context manager, just like ipywidgets' output
+widget.
+
 ```python
 from sidecar import Sidecar
 from ipywidgets import IntSlider
@@ -27,3 +28,9 @@ sl = IntSlider(description='Some slider')
 with sc:
     display(sl)
 ```
+
+When a single output is displayed in a Sidecar, it is allowed to occupy all of
+the vertical space available. If more content is displayed, the natural height
+is used instead.
+
+![sidecar](sidecar.gif)