Clarify default settings and which plugins are [ON].

[hugolpz]

As I do the recommended :

gulp.task('default', function () {
    return gulp.src('logo.svg')
        .pipe(svgmin())
        .pipe(gulp.dest('./out'));
});

I get an heavily lightened svg (great!!) without knowing which of #of the SVGO plugins were ON and working. As a first-time users, I want to know what is on/off at normal usage. The current tutorial (README.md) would gain to explain or link to the default settings relative to these plugins.

[ben-eb]

This is better documented upstream; svgo has a lot of dependents. Would be happy to include a link in this project's readme.

svg/svgo#472