BUG: vtkNRRDReader filename ambiguity. Fixes #3766

Slicer has a class vtkNRRDReader in Libs/vtkTeem. VTKv4 has a class
vtkNrrdreader in VTK/IO/Image. On case insensitive file systems
(e.g. Windows and some Macs), the statement #include "vtkNRRDReader.h"
will actually include the file vtkNrrdReader (from VTKv6). The
compiler will report the error: error: ‘vtkNRRDReader’ was not
declared in this scope.

CLI's that explicitly include vtkNRRDReader will have these
errors.
Prior to check-in:
https://github.com/Slicer/Slicer/commit/f0f0df61038b9591e895ccb4a40256167e5366d3
a cmake statement:
include_directories(BEFORE ${vtkTeem_INCLUDE_DIRS})
insured that the Slicer vtkNRRDReader would be found.

This patch restores the
include_directories(BEFORE ${vtkTeem_INCLUDE_DIRS})
with a more descriptive comment to prevent its removal in the future.

From: Bill Lorensen <bill.lorensen@gmail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23425 3bd1e089-480b-0410-8dfb-8563597acbee