Squashed 'machine/' changes from a0d8669..85d9e60

85d9e60 update readme
24fa672 can specify host commit id
a2c8aef update readme

git-subtree-dir: machine
git-subtree-split: 85d9e607f9f42c29d0c11308b74728f590955b5d
Merge commit 'cbd814d0df5c096473e4f042733228c16e44a1af'