add comment of dependency on FFTW3

Author: PrometheusPi

README.md

@@ -30,3 +30,12 @@ Software License
 *Clara2* is licensed under the **GPLv3+**. You can use any of our *libraries* with
 **GPLv3+ or LGPLv3+** (they are *dual-licensed*).
 Please refer to our [LICENSE](LICENSE)
+
+
+Dependency
+----------
+
+*Clara2* uses the FFTW library when used with the (faster) fft detector.
+If you install *Clara2*, you need to install FFTW as well. You can finde 
+compiled code at: http://www.fftw.org/ or the source code at 
+https://github.com/FFTW/fftw3. FFTW3 is under GNU General Public License v2.0.

src/include/makefile

@@ -21,6 +21,8 @@
 
 CC = g++
 CFLAGS = -Wall -O3 -c
+# compile against fftw library
+# http://www.fftw.org/ - an open source FFT library under GPL 2.0 license
 CFFT = -lfftw3 -lm
 
 all: libDetector.a fileExists.o

src/makefile

@@ -22,6 +22,8 @@
 
 CC = g++
 CFLAGS = -Wall -O3
+# compile against fftw library
+# http://www.fftw.org/ - an open source FFT library under GPL 2.0 license
 CFFT = -lfftw3 -lm
 COBJ = -c
 OMP = -fopenmp