Added example to the lxc_conf argument in the start command

Clarifies documentation for the `lxc_conf` argument in the `start` command. The documentation says the parameter is a dict; however, it requires a specific syntax such as:

```
[{"Key":"lxc.cgroup.cpu.shares","Value":"1"}, {"Key": ..., "Value": }]`
```

Related to dotcloud/docker-py#79