Document by default disabled cssnano transforms

[nene]

After lots of fiddling with cssnano, I finally looked at the css-loader source code to discover that some of the options are disabled by default:

    if(minimize) {
        var minimizeOptions = assign({}, query);
        ["zindex", "normalizeUrl", "discardUnused", "mergeIdents", "reduceIdents"].forEach(function(name) {
            if(typeof minimizeOptions[name] === "undefined")
                minimizeOptions[name] = false;
        });
        pipeline.use(cssnano(minimizeOptions));
    }

I managed to track this feature down to a commit titled Add useful minimize options.

Would be nice when the README also documented that some transforms are disabled, and explain why.

[SpaceK33z]

PR would be welcome.