Document color_threshold argumnet to create_dendrogram

[jonmmease]

In the not-yet-released plotly.py 3.4 there will be a new color_threshold argument to create_dendrogram to control the dendrogram cut height at which clusters are created.

See plotly/plotly.py#1214 for an example.

I think we could just add this example to https://plot.ly/python/dendrogram/

[cldougl]

thanks @jonmmease ! could you ping this issue when 3.4 is released?