Update README.md

[gabriel-curtino]

Hi!

This is a humble contribution. With all the colors definitions available will save time for others and it may makes a bit easier to understand and handle all Bulma variables.

[wtho]

Hey!
Great idea, but I think referencing a list in the bulma code would be better, so we don't have to maintain it for every version on our own.
Can you reference from where you took the definitions, so I can keep track of it in the future?

[gabriel-curtino]

That's true.

I've taken them from the documentation: https://bulma.io/documentation/customize/variables/