Uninstall instructions for Chef DK. 

Fixes https://github.com/opscode/chef-dk/issues/17.

@lamont-granquist can you check my *nix terminology? :smile: 
Per `man xargs`:

```
If this option is specified, xargs will use the data read from standard input to replace the first occurrence of replstr ........
```

`-J` only changes the first occurence of `%`. However using % is a better idea so I'll use that instead of `'{}'`.