Updated documentation for Windows installation

There wasn't any specific documentation to get Growl for Windows to work so after some searching for issues i came across this [comment](https://github.com/dylang/grunt-notify/issues/49#issuecomment-31238032) but did not have a clue how to do this. After some Googling and adding things to system PATH, grunt-notify now works beautifully.

The aim for this pull request is to update the document so it clearly states that additional steps are required for Windows. It also shows a screen capture for the process of adding a PATH to Windows.