Update usage section in readme

For reference, here's the usage text from `knife backup --help`:

```bash
$ knife backup --help
FATAL: Cannot find sub command for: 'backup --help'
Available backup subcommands: (for details, knife SUB-COMMAND --help)

** BACKUP COMMANDS **
knife backup export [COMPONENT [COMPONENT ...]] [-D DIR] (options)
knife backup restore [COMPONENT [COMPONENT ...]] [-D DIR] (options)
```