[docs] Document static getStyles() for theme systems

[justinfagnani]

See https://github.com/Polymer/lit-element/pull/866

Apps and design systems that want to add additional styles to a component should override getStyles(). Previously they might have tried to override finalize(), but style gathering has been moved from there.

[ianes1978]

i don't know if is usefull for you problem i have made a little module lit-css-global-scope to import in the lit-element component the global style and preserve the shadowdom styles