document jlab extension (closes #17)

Author: nicolaskruchten

README.md

@@ -9,3 +9,7 @@ Documentation and examples can be found at https://plotly.github.io/plotly_expre
 Just running `pip install plotly_express` in your terminal!
 
 Plotly Express is compatible with Python 3+ and Python 2.7.
+
+### Running in JupyterLab
+
+Using Plotly Express in JupyterLab requires the [`plotly-extension`](https://github.com/jupyterlab/jupyter-renderers/tree/master/packages/plotly-extension) to be installed by running `jupyter labextension install @jupyterlab/plotly-extension`.