Update README for new map feature

[eswat2]

New options:

• map
• alphas
• prefix (optional)

By default, it adds fallback values for the colors defined by clrs

Caveats:

1. If prefix is clrs, it merges your map colors into the default color map for clrs...
2. If you specify a different prefix, it only uses the map you provide...

...

[eswat2]

Alphas:

• array of 2 digit numbers
• range: 10-99

Should there be an explicit range check in the code ??

Example:

• #f0050 == red at 50% opacity
• var(--clrs-red-a50, #f0050)