Documentation: update Ubuntu installation
Documentation: update Ubuntu installation

The documentation was unclear as to where to get the `install-rkt.sh`
script. This makes it clear, and also describes how to install without
having to clone the entire rkt repo

sstabellini/rkt,alban/rkt,Shashwatsh/rkt,jakon89/rkt,cab105/rkt,kinvolk/rkt,rkt/rkt,kinvolk/rkt,lucasallan/rkt,joshix/rkt,coreos/rkt,dgonyeo/rkt,yifan-gu/rocket,ybubnov/rkt,squeed/rkt,Shashwatsh/rkt,lucasallan/rkt,ybubnov/rkt,joshix/rkt,yifan-gu/rocket,cab105/rkt,squeed/rkt,yifan-gu/rkt,dgonyeo/rkt,achanda/rkt,coreos/rkt,dgonyeo/rkt,achanda/rkt,jonboulle/rkt,shobhit85/rkt,intelsdi-x/rkt,nhlfr/rkt,kinvolk/rkt,rkt/rkt,jakon89/rkt,joshix/rkt,lucasallan/rkt,ybubnov/rkt,sstabellini/rkt,alban/rkt,rkt/rkt,joshix/rkt,nhlfr/rkt,alban/rkt,Shashwatsh/rkt,rkt/rkt,intelsdi-x/rkt,intelsdi-x/rkt,jonboulle/rkt,yifan-gu/rocket,lucasallan/rkt,sstabellini/rkt,achanda/rkt,shobhit85/rkt,cab105/rkt,shobhit85/rkt,kinvolk/rkt,yifan-gu/rkt,yifan-gu/rkt,nhlfr/rkt,jakon89/rkt,coreos/rkt,jonboulle/rkt,squeed/rkt