Add `bundle install` step to the README

When getting the project up and running for the first time, I got the following error after performing the steps in the README:
```
rake aborted!
cannot load such file -- fuburake
```
I think there's a `bundle install` step missing from the README that I at least had to run before `rake` would do anything meaningful. This pull request adds that step and a couple of other touches to the README file.