README: document sanity check skip
README: document sanity check skip

The variable that must be set to disable the virtualization distro
feature check was not documented in the README ... making it not
so useful.

Signed-off-by: Bruce Ashfield <284cc737d174a260eec2a2a6324a304f84b1ae0f@windriver.com>

lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization