README: add RGB color conversion example

[stokito]

No description provided.

[boronine]

The Color constructor example seems useful but the double[] rgb line is redundant I think

[stokito]

Sorry, when copied I removed the line. I updated the PR.

Also it turned out that this is not enough for the java.awt.Color because sometimes the rgb_r may be negative and the Color(float, float, float) will throw an exception on the bad range IllegalArgumentException: Color parameter outside of expected range: Red.

Here is a sample:

        HsluvColorConverter conv = new HsluvColorConverter();
        conv.hsluv_s = 100;
        conv.hsluv_l = 50;
        conv.hsluv_h = 128; // 128...255
        conv.hsluvToRgb();
        System.out.println(conv.rgb_r); //=> -2.2506760344320753E-13

This is a very small value but still it's negative.

So in fact it needs for an additional rounding and most implementations do have it, e.g. Conversations:

        return Color.rgb(
                (int) Math.round(converter.rgb_r * 255),
                (int) Math.round(converter.rgb_g * 255),
                (int) Math.round(converter.rgb_b * 255));

Here it uses conversion to int with rounding that hides the negative value to just 0. But in my case I wanted to use the float value directly and that's why I got the exception.

So could you add a check to avoid negative values? Or maybe there are also possible values bigger than upper range. This was a difficult to catch and understand for me because I never worked with colors and faced the exception in runtime after some testing. It might be a bug on production.

#3

[boronine]

Looks good, thanks!