Splits the ideas scaffold instructions.

This expands the instructions to create an `ideas` scaffold in the example app, mostly to overcome an issue that many attendees had (at least in the group I was in and mostly on windows): while pasting the scaffold generation command, they would also paste the subsequent `rake db:migrate` on the same line (probably an issue with line breaks), generating a broken scaffold.

By splitting the instructions into two, we can mitigate the issue.

I'm attaching a screenshot of the new version for ease of review.

![scaffold](https://f.cloud.github.com/assets/537608/1276337/4156e5c6-2e5d-11e3-8faa-c2915c4865a4.png)