README.md: Clarify how colors can be specified

While technically one can specify colors otherwise, for the sake of simplicity,
mention only `CRGB`, but do mention it explicitly, so the reader does not need
to look at the example to figure this out.

Fixes #9.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

README.md

@@ -68,7 +68,8 @@ The plugin provides the following effects:
 ### `Haunt([color])`
 
 > A ghostly haunt effect, that trails the key taps with a ghostly white color
-> (or any other color, if specified).
+> (or any other color, if specified). Use the `CRGB(r,g,b)` macro to specify the
+> color, if you want something else than the ghostly white.
 
 ### `BlazingTrail()`