[vtkusers]ask how to read DICOM(again): FAQ added

[Mathieu Malaterre]

Since the question comes often on the mailing list, I added a FAQ entry for it:

http://www.vtk.org/Wiki/VTK_FAQ#The_spacing_in_my_DICOM_files_are_wrong

let me know if it make any sense.

-- 
Mathieu