README: add details for astral mode
README: add details for astral mode

BeardandFedora/xregexp,GerHobbelt/xregexp,mathiasbynens/xregexp,cesarmarinhorj/xregexp,slevithan/xregexp,mathiasbynens/xregexp,imvijaybala/xregexp,mathiasbynens/xregexp,exexzian/xregexp,vsimonian/xregexp,kod3r/xregexp,kod3r/xregexp,Iced-Tea/xregexp,imvijaybala/xregexp,Iced-Tea/xregexp,vsimonian/xregexp,GerHobbelt/xregexp,Akylas/xregexp,GerHobbelt/xregexp,slevithan/XRegExp,exexzian/xregexp,slevithan/xregexp,sagarrakshe/xregexp,carabina/xregexp,slevithan/XRegExp,sagarrakshe/xregexp,cesarmarinhorj/xregexp,Akylas/xregexp,carabina/xregexp,slevithan/xregexp,mathiasbynens/xregexp