Added note that FFTW v3 or later is required.

docs/user-guide/installation/source.tex

@@ -86,7 +86,7 @@ From a terminal:
     \begin{notebox}
     FFTW and Arpack are optional. They provide additional or optimised
     functionality, but alternative implementations are included as part of
-    \nekpp.
+    \nekpp. Note that FFTW version 3 or later is required.
     \end{notebox}
 
     \item Change into the source-code directory, create a \inltt{build}