title: Add title to the plotly legend
body: <p>In the following example how can i add a title to the legend in plot_ly for R ?</p>
  
  <pre><code>mtcars %&gt;%   plot_ly(x = ~disp, y = ~mpg, color = ~factor(cyl), size = ~wt) %&gt;%   add_markers(
      hoverinfo = "text",
      text = ~paste("Displacement = ", disp, "\nMiles Per Gallon = ", mpg)   ) %&gt;%   layout(title ="Custom Hover Text")
  </code></pre>
  
  <p>thanks</p>