Merge pull request #7037 from plotly/marthacryan-patch-1

Update CONTRIBUTING.md

Author: marthacryan

CONTRIBUTING.md

@@ -83,6 +83,8 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 - [`npm`](https://www.npmjs.com/) v7.x and up to ensure that the
   [`package-lock.json`](https://docs.npmjs.com/files/package-lock.json) file is
   used and updated correctly.
+- [`python3`](https://www.python.org/downloads/)
+Note: for M1 mac users specifically, you might need to install some [extra dependencies](https://github.com/Automattic/node-canvas/issues/1733#issuecomment-761703018). 
 
 #### Step 1: Fork the plotly.js repository, clone your fork and step into it