complete GitFlow.init with warning messages

Signed-off-by: Sebastien Varrette <sebastien.varrette@uni.lu>
complete Git.subtree_init?

Signed-off-by: Sebastien Varrette <sebastien.varrette@uni.lu>
document subtree/submodules

Signed-off-by: Sebastien Varrette <sebastien.varrette@uni.lu>