Update CONTRIBUTING.md

Author: marthacryan

CONTRIBUTING.md

@@ -169,6 +169,26 @@ and
 [`pip`](https://pip.pypa.io/en/stable/reference/pip_install/#install-editable)
 documentation on _development mode_.
 
+### Updating the `js/` directory
+**This is only necessary if you're making changes to the `js/` directory.**
+To rebuild the javascript, you can run `npm install && npm run build` from the `js/` directory. 
+
+**Notes on the contents of the `js/` directory:**
+The `js/` directory contains code to run plotly in Jupyter notebooks. It is shipped
+as part of the python package, so the build files are located in the `plotly/` directory
+under `plotly/labextension`. The files in that directory are committed so that if you
+aren't making a change to the `js/` directory, you don't have to worry about building it.
+There are two kinds of Jupyter support included in the `js/` directory:
+1. **Widget**: This is a more interactive model for rendering plotly in notebooks. You can use this
+through `FigureWidget`. It allows for communication between the javascript frontend and the python backend,
+but it has the drawback of requiring an additional package (`anywidget`). The code for this is located under
+`js/src/widget.ts`, and is included in the python package through `plotly/package_data/widgetbundle.js`. 
+2. **Mime Renderer**: This is a less interactive model. The plot is rendered but can't send any
+information back to the python side. For most people this is enough, so this is the default
+renderer for the `Figure` class. The mime renderer is used automatically by JupyterLab / Jupyter Notebook
+when it sees the mimetype `application/vnd.plotly.v1+json` in the notebook output. We include a JupyterLab extension in our python package that loads `plotly.js` and renders it when that mimetype is found in notebook
+output. This allows for us to avoid having to embed `plotly.js` in the notebook output. The code for this is found under `js/src/mimeExtension.ts` and is compiled into `plotly/labextension` through `jupyter labextension build`, which is included in `npm run build`. 
+
 ### Configure black code formatting
 
 This repo uses the [Black](https://black.readthedocs.io/en/stable/) code formatter,