add download links to README.md
add download links to README.md

both `http:` and `https:` are supported

datawolf/os,juliengk/os,hairyhenderson/rancheros,fentas/os,juliengk/os,1yvT0s/os,duguhaotian/os,ibuildthecloud/os,rancherio/os,maxfierke/rancher-os,datawolf/os,OnePaaS/os,duguhaotian/os,luxas/os,fentas/os,rowhit/os,maxfierke/rancher-os,rancherio/os,gpndata/os,rowhit/os,imikushin/os,maxfierke/rancher-os,rowhit/os,rancherio/os,luxas/os,OnePaaS/os,gizmotronic/os,OnePaaS/os,luxas/os,rancher/os,datawolf/os,1yvT0s/os,SvenDowideit/os,gpndata/os,hairyhenderson/rancheros,imikushin/os,gizmotronic/os,hairyhenderson/rancheros,ibuildthecloud/os,gpndata/os,rancher/os,SvenDowideit/os,1yvT0s/os,fentas/os,duguhaotian/os