Create a ciao-down install doc

[jodh-intel]

Since ciao-down now support CC 3.0, we should produce an installation doc for parity with:

• https://github.com/01org/cc-oci-runtime/blob/master/documentation/Quickstart-Clear-Containers-in-a-Ubuntu-VM.md

[grahamwhaley]

@mcastelino noted that as the 'native' cc3 install in ciao-down will bit-rot quite fast, it will be better to document that the install script can be run in a native ciao-down to get a fresh, recent CC3 installed :-)

[grahamwhaley]

Notably scripts from here: https://github.com/clearcontainers/tests/tree/master/integration/kubernetes