chore(README): add gitter badge
chore(README): add gitter badge

avigad/lean,johoelzl/lean,Kha/lean,rlewis1988/lean,Kha/lean,dselsam/lean,johoelzl/lean,fpvandoorn/lean,leodemoura/lean,leanprover/lean,rlewis1988/lean,dselsam/lean,leanprover/lean,rlewis1988/lean,digama0/lean,avigad/lean,fpvandoorn/lean,rlewis1988/lean,leanprover/lean,leanprover-community/lean,leanprover/lean,dselsam/lean,digama0/lean,leanprover/lean,fpvandoorn/lean,leodemoura/lean,johoelzl/lean,avigad/lean,leodemoura/lean,leanprover-community/lean,leanprover-community/lean,Kha/lean,rlewis1988/lean,dselsam/lean,Kha/lean,digama0/lean,Kha/lean,leanprover-community/lean,Kha/lean,avigad/lean,johoelzl/lean,digama0/lean,avigad/lean,leodemoura/lean,leanprover-community/lean,rlewis1988/lean,dselsam/lean,johoelzl/lean,fpvandoorn/lean,leodemoura/lean,leanprover-community/lean,digama0/lean,dselsam/lean,digama0/lean,fpvandoorn/lean,fpvandoorn/lean,leodemoura/lean,johoelzl/lean,avigad/lean,leanprover/lean