README: Update slack reference to CNCF
README: Update slack reference to CNCF

Signed-off-by: Mauricio Vásquez <95f407c1ffa7689a0a3f2f21b1bf582fe8359351@kinvolk.io>

dmcgowan/containerd,estesp/containerd,mikebrow/containerd,thaJeztah/containerd,vdemeester/containerd,dmcgowan/containerd,estesp/containerd,containerd/containerd,thaJeztah/containerd,mikebrow/containerd,vdemeester/containerd,containerd/containerd