changed readme for function hsl() where parameter in documentation are not described like implemented

Documentation of function hsl(hue, saturation, brightPercent) updated,
because implementation has changed. Input value for hue are degrees
between 0 and 359 (°). Input value for saturation is percent between 0
and 100 (%). Re-calculation for these inputs will be done by
_getHSLState in lightstate.js