Added empty hubot-scripts.json file and added section to Readme on hubot scripts

Took me a bit to figure out how to install Hubot scripts. Figured I'd add some boilerplate so others don't get confused.

We could also go ahead and add the "hubot-scripts" dependency to the package.json, but I figured there was probably a reason it's not there already and we don't necessarily want to make people install all the scripts from the dependency into node_modules if they're not going to use them.

But if you want me to add that to the package.json by default, let me know and I'll update the pull request.