Add graph_objs performance considerations section for plotly.py

[jonmmease]

See discussion in plotly/plotly.py#1052.

Basically we should explain that there is some performance overhead in using the graph_objs hierarchy due to the validation logic. And if you know that you know what you're doing things will be faster to construct if you just use dicts and call plot/iplot with validate=False.

This can be particularly painful when building up animations with many frames, as was the case in plotly/plotly.py#1052.