Advise about vagrant-cachier in the README
Advise about vagrant-cachier in the README

Change-Id: Id3857481cac17540e02420849f0b5fc83339a2ab

meniy/Vagrant,openstack-dev/devstack-vagrant,meniy/Vagrant,meniy/Vagrant,olivierlemasle/devstack-vagrant,olivierlemasle/devstack-vagrant,unibg-seclab/devstack-vagrant,AlessandroCopler/TestVagrant,unibg-seclab/devstack-vagrant,AlessandroCopler/TestVagrant,openstack-dev/devstack-vagrant,unibg-seclab/devstack-vagrant