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

platform.vcxproj PreBuildEvent has been tweaked 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)
Merge pull request #333 from krustyreturns/helix_xbmc_sln

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