Added additional notes for review.ninja

you also have to ensure the review.ninja repository is installed on the same root as vagrant.review.ninja otherwise you get the following error when running ``vagrant up``:

```
Bringing machine 'default' up with 'virtualbox' provider...
There are errors in the configuration of this machine. Please fix
the following errors and try again:

vm:
* The host path of the shared folder is missing: ../review.ninja
```