This pull request adds support for GhostDoc to .gitignore

Greetings - This pull request adds support to ignore a Visual Studio plugin called GhostDoc (Details about GhostDoc [here](http://submain.com/products/ghostdoc.aspx)).  GhostDoc creates *.GhostDoc.xml files that wind up being a pain to deal with and I've found myself issues git rm *.GhostDoc.xml files more than once so I decided to add it to my .gitignore and then figured I'd share.

Hope this works out for others in the community that might be using this awesome plugin.