chore(README.md): add link to standard library documentation
chore(README.md): add link to standard library documentation

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

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