docs: Add a note how to do stable branch cherry-picking
docs: Add a note how to do stable branch cherry-picking

Stable branches were introduced. This change will add a small hint on how
patches can be picked into stable branches.

Signed-off-by: Fabian Deutsch <de69c509e52dfe544d526a81279fe21e1252748c@fedoraproject.org>

kubevirt/kubevirt,davidvossel/kubevirt,stu-gott/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt,davidvossel/kubevirt,stu-gott/kubevirt,kubevirt/kubevirt,stu-gott/kubevirt,stu-gott/kubevirt,davidvossel/kubevirt,davidvossel/kubevirt