Adding libfftw3 dependency in the documentation

VERSION

-ltfatpy:1.0.4
+ltfatpy:1.0.5
 ltfat:2.1.0

doc/install.rst

@@ -28,7 +28,8 @@ For developpers only:
 Scientific Python libraries
 ---------------------------
 
-**ltfatpy** is using **numpy** >= 1.9, **scipy** >= 0.15 and **matplotlib** >= 1.4. For installing those packages read the instructions in:
+* You need to have **libfftw3**, development version, installed. For debian based linux systems use ``$apt install libfftw3-dev``. For other systems, please read the documentation of `fftw <http://www.fftw.org/>`_.
+* **ltfatpy** is using **numpy** >= 1.9, **scipy** >= 0.15 and **matplotlib** >= 1.4. For installing those packages read the instructions in:
 `here <http://www.scipy.org/install.html>`_.
 
 Downloading **ltfatpy**