Python Dendogram doc - add 'color_threshold' usage#1231

Merged

priyatharsan merged 1 commit intosource-design-mergefrom

dendogram-color-threshold

Jan 30, 2019

Commits on Jan 10, 2019

closes https://github.com/plotly/documentation/issues/1130
priyatharsan
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