Add color reference table and updated swatch image to README

[CampAsAChamp]

Adds color reference table for easy reference and copy & paste

All 19 colors from autoload/onedark.vim listed with hex, 256-color, and 16-color values in inline code blocks for easy copy/paste.

• Adds more colors to the swatch image to match the actual colors in the file
• Swatch order matches the table — colors flow left-to-right across rows (background, foreground, red, then green, yellow, blue, …) so the two references read consistently.