add stamen example

Author: LiamConnors

doc/python/mapbox-layers.md

@@ -201,6 +201,24 @@ fig.update_layout(margin={"r":0,"t":0,"l":0,"b":0})
 fig.show()
 ```
 
+<!-- #region -->
+#### Stamen Watercolor using a Custom Style URL
+
+Here's an example of using a custom style URL that points to the Stadia Maps service to use the `stamen_watercolor` base map.
+
+```python
+import pandas as pd
+quakes = pd.read_csv('https://raw.githubusercontent.com/plotly/datasets/master/earthquakes-23k.csv')
+
+import plotly.graph_objects as go
+fig = go.Figure(go.Densitymap(lat=quakes.Latitude, lon=quakes.Longitude, z=quakes.Magnitude,
+                                 radius=10))
+fig.update_layout(map_style="https://tiles.stadiamaps.com/styles/stamen_watercolor.json?api_key=YOUR-API-KEY", map_center_lon=180)
+fig.update_layout(margin={"r":0,"t":0,"l":0,"b":0})
+fig.show()
+```
+<!-- #endregion -->
+
 <!-- #region -->
 ### Mapbox