Update README to provide info on building pvr addons together with XBMC ...

...on windows

platform.vcxproj PreBuildEvent is also modified to locate ConfigureAddons.bat using a relative path from the $(ProjectDir) variable so it works regardless of where the solution file lives
This makes it work when part of XBMC solution but it still continues to work when part of xbmc-pvr-addons solution as well...