docs: Remove note about library being in beta phase.
docs: Remove note about library being in beta phase.

rpavlik/luabind,alex85k/luabind,rpavlik/luabind,alex85k/luabind,Oberon00/luabind,lonski/luabind,rpavlik/luabind,luabind/luabind,luabind/luabind,mapbox/luabind,luabind/luabind,Oberon00/luabind,MrApplejuice/luabind,Kazade/luabind-deboostified,plsytj/luabind,grafi-tt/luabind,alex85k/luabind,Wohlhabend-Networks/luabind-deboostified,ltjax/luabind-deboostified,rpavlik/luabind,decimad/luabind-deboostified,galek/luabind-deboostified,davidar/luabind,mdave88/luabind,rpavlik/luabind,alex85k/luabind,luabind/luabind