chore(docs): update README related to Chroma prefixes

[xcoulon]

explain how to generate a CSS with custom prefixes using the Chroma CLI

Fixes #971

Signed-off-by: Xavier Coulon xcoulon@redhat.com

[codecov[bot]]

Codecov Report

Merging #1101 (ee5490a) into master (3ac35d0) will not change coverage. The diff coverage is n/a.

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##           master    #1101   +/-   ##
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  Coverage   80.79%   80.79%           
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  Files          89       89           
  Lines        7588     7588           
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  Hits         6131     6131           
  Misses        963      963           
  Partials      494      494