Adds stable MSI urls to README.rst
Adds stable MSI urls to README.rst

Change-Id: Ic38fd087b3a8ea2ba985e5a0cb06138504376eeb

ader1990/cloudbase-init,alexpilotti/cloudbase-init,chialiang-8/cloudbase-init,openstack/cloudbase-init,stackforge/cloudbase-init,stefan-caraiman/cloudbase-init,cmin764/cloudbase-init