Setup instructions incomplete

So I downloaded the hook into the right folder using wget, but it only works after you make it executable.

``` bash
$ wget https://raw.githubusercontent.com/team-taniwha/misc-scripts/master/prepare-commit-msg`
$ chmod +x prepare-commit-msg
```

It would be nice if the README mentioned that you need to ensure the hook is executable, otherwise it just silently doesn't work.
