BUG: Display the legend when calling plot with kind='density'. GH4216
Check if a plotting DataFrame with kind='kde' has a legend. GH4216
Updated release note about GH4216.

BUG: legend was not displayed in `DataFrame.plot(kind='kde')`.
Merge pull request #4221 from garaud/plot_density_legend

BUG: Display the legend when calling plot with kind='density'. GH4216