Add example for setNumEntries

Turns out you can already pass in arguments to the load function. So I simply added the example to the readme.

```
    # :call-seq:
    #   load(url, args = {})
    # downloads this feed from Google's servers
    # Arguments: http://code.google.com/apis/ajaxfeeds/documentation/reference.html#_fonje_load
    standard_api(:load){|h| h['feed']}
```