Readme: add note about installing coffee globally

`cake` is not available unless you have `coffee-script` installed globally. I edited the Readme file to take note of that.

Btw, have you considered having an `npm start` command instead? You can just edit package.json and add:

```js
  "scripts": {
    "run": "cake -w server"
  },
  "devDependencies": {
    "coffee-script": "^1.7.0"
  }
```

this will automatically use `cake` as provided in your *node_modules/*.