consider adding a bar plot with benchmark to README

[Polkas]

Create a code to run a benchmark in inst and compare styler and tergo.
The results will be presented in README file with reference to code in inst directory.
Possible run a few different scenarios.
I feel running them on dplyr/shiny/ggplot2 as a whole package can be a good idea.

Ps. The benchmark run has to be clear about machine on which it was run.

[Polkas]

Done here Polkas#2