Add a broad description of the API to README
Add a broad description of the API to README

Add a broad description of the API's object types to the top level
README, so that it's not just scattered around in man pages for the CLI
tool.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

containers/storage,rhatdan/storage,rhatdan/storage,containers/storage,containers/storage,rhatdan/storage