doc(examples/lean): add a new example with even/odd numbers
doc(examples/lean): add a new example with even/odd numbers

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

leanprover/lean,soonhokong/lean-osx,fgdorais/lean,soonhokong/lean-osx,avigad/lean,codyroux/lean0.1,leanprover/lean,digama0/lean,javra/lean,leanprover-community/lean,htzh/lean,soonhokong/lean,soonhokong/lean,dselsam/lean,eigengrau/lean,c-cube/lean,sp3ctum/lean,javra/lean,rlewis1988/lean,digama0/lean,eigengrau/lean,leodemoura/lean,htzh/lean,dselsam/lean,c-cube/lean,rlewis1988/lean,sp3ctum/lean,leanprover-community/lean,UlrikBuchholtz/lean,rlewis1988/lean,UlrikBuchholtz/lean,leanprover/lean,leanprover-community/lean,levnach/lean,dselsam/lean,soonhokong/lean,leanprover/lean,Kha/lean,levnach/lean,leanprover-community/lean,soonhokong/lean-windows,codyroux/lean0.1,UlrikBuchholtz/lean,digama0/lean,fgdorais/lean,johoelzl/lean,dselsam/lean,UlrikBuchholtz/lean,johoelzl/lean,javra/lean,fpvandoorn/lean,javra/lean,fpvandoorn/lean,sp3ctum/lean,htzh/lean,soonhokong/lean,Kha/lean,avigad/lean,rlewis1988/lean,javra/lean,rlewis1988/lean,leodemoura/lean,Kha/lean,digama0/lean,htzh/lean,leodemoura/lean,soonhokong/lean-windows,fpvandoorn/lean2,codyroux/lean0.1,htzh/lean,johoelzl/lean,leanprover-community/lean,fpvandoorn/lean2,sp3ctum/lean,fpvandoorn/lean2,fgdorais/lean,eigengrau/lean,soonhokong/lean,rlewis1988/lean,leanprover/lean,fpvandoorn/lean,soonhokong/lean-osx,Kha/lean,leanprover/lean,c-cube/lean,fpvandoorn/lean,avigad/lean,eigengrau/lean,johoelzl/lean,soonhokong/lean-osx,Kha/lean,fpvandoorn/lean,levnach/lean,soonhokong/travis_test,sp3ctum/lean,fpvandoorn/lean2,fgdorais/lean,soonhokong/lean-osx,fpvandoorn/lean,levnach/lean,avigad/lean,c-cube/lean,dselsam/lean,johoelzl/lean,fpvandoorn/lean2,johoelzl/lean,fgdorais/lean,soonhokong/travis_test,leodemoura/lean,eigengrau/lean,avigad/lean,soonhokong/lean-windows,digama0/lean,leanprover-community/lean,c-cube/lean,UlrikBuchholtz/lean,digama0/lean,Kha/lean,levnach/lean,soonhokong/travis_test,leodemoura/lean,codyroux/lean0.1,fgdorais/lean,soonhokong/travis_test,dselsam/lean,soonhokong/lean-windows,soonhokong/lean-windows,avigad/lean,leodemoura/lean