Move uncommon asset compilation instructions to dir
Move uncommon asset compilation instructions to dir

Most users will never edit or compile JS. Move the instructions so the
main README focuses on common use cases.

janisz/marathon-ui,watonyweng/marathon-ui,cribalik/marathon-ui,pierlo-upitup/marathon-ui,mesosphere/marathon-ui,Kosta-Github/marathon-ui,yp-engineering/marathon-ui,Raffo/marathon-ui,watonyweng/marathon-ui,quamilek/marathon-ui,mesosphere/marathon-ui,Kosta-Github/marathon-ui,quamilek/marathon-ui,cribalik/marathon-ui,yp-engineering/marathon-ui,pierlo-upitup/marathon-ui,Raffo/marathon-ui,janisz/marathon-ui