added documentation for new .colorless option

README.md

@@ -96,6 +96,7 @@ Progressive comes with *lots* of different options so that you are able to custo
 | .simple | make details, above the progress bar, disappear |
 | .counter | counter mode (= don't show a progress bar) |
 | .static | don't show animated spinner |
+| .colorless | don't use colors |
 | .ratioless | don't show progress as a ratio |
 | .keep | keep progress bar as-is after finishing, without minimizing |
 | .hide | hide progress bar completely, after finishing |