v4 notice

Author: nicolaskruchten

nb.tpl

@@ -15,6 +15,12 @@ language: python/next
 {{ '{% raw %}' }}
 <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.2/require.js"></script>
 
+<div style="margin: 20px auto; width: 75%; background: #f4f4f8; padding: 20px; font-weight: 400; line-height: 1.85; text-align: center;">
+	<strong>Note:</strong> this is <em>pre-release documentation</em> for the upcoming version 4 of Plotly.py.<br/>
+	See our <a href="https://plot.ly/python/next/getting-started" style="color: #2391fe">Getting Started</a> guide for v4 installation instructions,
+	or head on over to the <a href="https://plot.ly/python" style="color: #2391fe">current v3 docs</a>
+</div>
+
 {%- endblock header-%}