Better explanation of GPL consequences (JuliaMath#43)

README.md

@@ -17,7 +17,16 @@ The FFTW library will be downloaded on versions of Julia where it is no longer d
 as part of Julia.
 Note that FFTW is licensed under GPLv2 or higher (see
 [its license file](http:https://www.fftw.org/doc/License-and-Copyright.html)), but the bindings
-here are licensed under MIT.
+to the library in this package, FFTW.jl, are licensed under MIT.
+This means that code using the FFTW library via the FFTW.jl bindings is subject to FFTW's
+licensing terms.
+Code using alternative implementations of the FFTW API, such as
+[MKL's FFTW3 interface](https://software.intel.com/en-us/mkl-developer-reference-c-fftw3-interface-to-intel-math-kernel-library)
+are instead subject to the alternative's license.
+If you distribute a derived or combined work, i.e. a program that links to and is distributed
+with the FFTW library, then that distribution falls under the terms of the GPL.
+If you just distribute source code that links to FFTW.jl, and users have to download FFTW
+or MKL to provide the backend, then the GPL probably doesn't have much effect on you.
 
 [![Travis](https://travis-ci.org/JuliaMath/FFTW.jl.svg?branch=master)](https://travis-ci.org/JuliaMath/FFTW.jl)
 [![AppVeyor](https://ci.appveyor.com/api/projects/status/hofbdbyt287qn49s/branch/master?svg=true)](https://ci.appveyor.com/project/ararslan/fftw-jl/branch/master)