Added installation doc.
Added installation doc.

opsdisk/pyrax,briancurtin/pyrax,rackspace/pyrax,rackerlabs/heat-pyrax,vikomall/pyrax,EdLeafe/pyrax,pratikmallya/pyrax,emonty/pyos,roelvv/pyrax,0dataloss/pyrax,naemono/pyrax,sivel/pyrax,mandx/pyrax,ddaeschler/pyrax