README and platform.vcxproj update for building pvr addons together with XBMC on windows

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...