Mention when not to use bar charts to avoid common pitfalls

[joelostblom]

From #160 (comment)

Note: Although bar charts are often used to display mean and median values,
they are generally a poor choice for these types of visualizations and should be avoided.
The reason for this is that they hide all the variation in the data and only show a single value.
Instead it's better to show the distribution of all the individual data points
(potentially together with an additional visual mark for the mean or median).

More info here on why this is important and can be misleading https://pages.github.ubc.ca/mds-2022-23/DSCI_531_viz-1_students/lectures/2-data-types_graphical-marks_visual-encondings.html#bar-charts