Fix doc: using yo to create hubot

now `hubot --create` is deprecated:

```
$ hubot --create
'hubot --create' is depreated. Use the yeoman generator instead:
    npm install -g generator-hubot
    mkdir yourbot
    yo hubot
See https://github.com/github/hubot/blob/master/docs/README.md for more details on getting started.
```

I tried above message, but couldn't.
Instead, installing `generator-hubot-bot` and `yo hubot-bot` is good in my environment, so i wrote them to README.md.

thanks.