Document "Why not just use a plotting library like mpl directly?"

[danielballan]

Why not just use a plotting library (like matplotlib) directly?

1. Portability between frameworks. Writing the plot-building code in a framework-agnostic way makes it effortlessly portable between all the frameworks supported by bluesky-widgets, including Jupyter, Qt, "headless" image file generation, and more to come.
2. Brevity and easy of use for targeted use cases. The ploit-builders enable succinct ways to specify plots. It's all still just Python code, but it requires less familiarity with scipy libraries and Bluesky data structures.
3. Streaming is handled for you. Making live-updating plots work right is detailed-oriented work, and this just handles it for you.

Also, when viewing data using a given framework, you still have access to the framework directly. You are free to go around bluesky-widgets when you want to do things that it does not know how to do.