Mention GPL in the README (#41)

Fixes #39.

README.md

@@ -4,6 +4,12 @@ This package provides Julia bindings to the [FFTW](http://www.fftw.org/) library
 fast Fourier transforms, as well as functionality useful for signal processing.
 These functions were formerly a part of Base Julia.
 
+The FFTW library will be downloaded on versions of Julia where it is no longer distributed
+as part of Julia.
+Note that FFTW is licensed under GPLv2 or higher (see
+[its license file](http://www.fftw.org/doc/License-and-Copyright.html)), but the bindings
+here are licensed under MIT.
+
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