Legend of colors of the examples in the README.md

The README mentions the types of characters, but does not correlate them to the colors and the "icons" used in the examples.

It would be nice starting of the Preview section with this information.

edit: https://github.com/sharkdp/hexyl/pull/235 added hexyl --print-color-table, which maybe can be used to auto-generate this info into the README.md.