doc(README.md): group build instructions by system
doc(README.md): group build instructions by system

fgdorais/lean,levnach/lean,c-cube/lean,javra/lean,avigad/lean,fpvandoorn/lean,UlrikBuchholtz/lean,UlrikBuchholtz/lean,soonhokong/lean-osx,dselsam/lean,dselsam/lean,fpvandoorn/lean2,javra/lean,leanprover/lean,leanprover-community/lean,c-cube/lean,avigad/lean,fgdorais/lean,fpvandoorn/lean2,avigad/lean,digama0/lean,javra/lean,leodemoura/lean,leanprover-community/lean,johoelzl/lean,fgdorais/lean,fpvandoorn/lean,leanprover-community/lean,Kha/lean,avigad/lean,rlewis1988/lean,leodemoura/lean,soonhokong/lean-osx,Kha/lean,digama0/lean,avigad/lean,leodemoura/lean,sp3ctum/lean,fpvandoorn/lean,UlrikBuchholtz/lean,rlewis1988/lean,johoelzl/lean,htzh/lean,fpvandoorn/lean2,dselsam/lean,rlewis1988/lean,fgdorais/lean,rlewis1988/lean,htzh/lean,soonhokong/lean,johoelzl/lean,Kha/lean,digama0/lean,fgdorais/lean,soonhokong/lean,avigad/lean,eigengrau/lean,rlewis1988/lean,soonhokong/lean-osx,fpvandoorn/lean,eigengrau/lean,Kha/lean,fgdorais/lean,c-cube/lean,leanprover-community/lean,johoelzl/lean,fpvandoorn/lean2,johoelzl/lean,leanprover/lean,digama0/lean,soonhokong/lean-windows,soonhokong/lean-windows,soonhokong/lean-osx,johoelzl/lean,htzh/lean,dselsam/lean,dselsam/lean,levnach/lean,fpvandoorn/lean,rlewis1988/lean,soonhokong/lean-osx,soonhokong/lean-windows,Kha/lean,sp3ctum/lean,c-cube/lean,soonhokong/lean,htzh/lean,soonhokong/lean-windows,UlrikBuchholtz/lean,dselsam/lean,levnach/lean,eigengrau/lean,leodemoura/lean,fpvandoorn/lean2,levnach/lean,sp3ctum/lean,javra/lean,sp3ctum/lean,leanprover/lean,leanprover/lean,soonhokong/lean,eigengrau/lean,leanprover-community/lean,c-cube/lean,digama0/lean,htzh/lean,soonhokong/lean-windows,sp3ctum/lean,UlrikBuchholtz/lean,leanprover/lean,leodemoura/lean,leanprover-community/lean,eigengrau/lean,levnach/lean,leodemoura/lean,fpvandoorn/lean,Kha/lean,soonhokong/lean,javra/lean,leanprover/lean,digama0/lean