Add a table showing the different colors and their usecases

[spcbfr]

Hello,
Rather then having to always refer to the code for colors, a table in the readme with each color name, hex value and usecase would be nice, this would make porting the theme so much easier

here is an example:

[antoineco]

This was partially addressed in:

• Add palette previews #164
• Create Palette overview #149

The color semantics table is still missing, but we should be able to effortlessly reuse the work done in Everforest (both colorschemes have a lot in common).