Mention Preferences in installation docs (#192)

docs/src/index.md

@@ -15,11 +15,12 @@ Pkg.add("FFTW")
 
 from the Julia REPL.
 
-Users with a build of Julia based on Intel's Math Kernel Library (MKL) can take use MKL
-for FFTs by setting an environment variable `JULIA_FFTW_PROVIDER` to `MKL` and running
-`Pkg.build("FFTW")`.
-Setting this environment variable only needs to be done for the first build of the package;
-after that, the package will remember to use MKL when building and updating.
-Note however that MKL provides only a subset of the functionality provided by FFTW. See
+Users with a build of Julia based on Intel's Math Kernel Library (MKL) can use MKL
+for FFTs by setting a preference in their top-level project by either using the
+`FFTW.set_provider!()` method, or by directly setting the preference using
+[`Preferences.jl`](https://github.com/JuliaPackaging/Preferences.jl).  Note that this
+choice will be recorded for the current project, and other projects that wish to use
+MKL for FFTs should also set that same preference.
+Note further that MKL provides only a subset of the functionality provided by FFTW. See
 Intel's [documentation](https://software.intel.com/en-us/mkl-developer-reference-c-using-fftw3-wrappers)
 for more information about potential differences or gaps in functionality.