state more precisely 64bit fftw3 usage

visualc/how-to-fftw.txt

@@ -1,9 +1,10 @@
-../../fftw-3.3.4-dll32 is the place to deploy the 32 bit version
-of the fftw3 library under VC++ which can be found here:
+../../fftw-3.3.4-dll32 and ../../fftw-3.3.4-dll64
+are the places to deploy the appropriate 32/64 bit version
+of the fftw3 libraries under VC++ which can be found here:
 
  http://www.fftw.org/install/windows.html
 
-More precisely,
+More precisely (here in case of the 32 bit version),
  - you have to create the directory ../../fftw-3.3.4-dll32
     (which is side by side and thus outside of the ngspice source tree)
  - you have to fetch fftw-3.3.4-dll32.zip and deploy it there
@@ -17,8 +18,8 @@ At least you need to unpack these three files
 Then use vngspice-fftw.vcxproj to compile ngspice
 
 If you need to distribute ngspice.exe to another directory,
-copy libfftw3-3.dll into the same directory or to a place
-which is in your PATH environment.
+copy the appropriate libfftw3-3.dll into the same directory or 
+to a place which is in your PATH environment.
 
 ----