Make note of the need to overload *

Also reformat the README to wrap long lines.

Author: ararslan

README.md

@@ -9,20 +9,31 @@ Documentation:
 [![](https://img.shields.io/badge/docs-stable-blue.svg)](https://JuliaMath.github.io/AbstractFFTs.jl/stable)
 [![](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaMath.github.io/AbstractFFTs.jl/latest)
 
-This package is mainly not intended to be used directly.  Instead, developers of packages that implement FFTs (such as [FFTW.jl](https://github.com/JuliaMath/FFTW.jl)) extend the types/functions defined in `AbstractFFTs`.  This multiple FFT packages to co-exist with the same underlying `fft(x)` and `plan_fft(x)` interface.
+This package is mainly not intended to be used directly.
+Instead, developers of packages that implement FFTs (such as [FFTW.jl](https://github.com/JuliaMath/FFTW.jl))
+extend the types/functions defined in `AbstractFFTs`.
+This allows multiple FFT packages to co-exist with the same underlying `fft(x)` and `plan_fft(x)` interface.
 
 ## Developer information
 
 To define a new FFT implementation in your own module, you should
 
-* Define a new subtype (e.g. `MyPlan`) of `AbstractFFTs.Plan{T}` for FFTs and related transforms on arrays of `T`.  This must have a `pinv::Plan` field, initially undefined when a `MyPlan` is created, that is used for caching the inverse plan.
+* Define a new subtype (e.g. `MyPlan`) of `AbstractFFTs.Plan{T}` for FFTs and related transforms on arrays of `T`.
+  This must have a `pinv::Plan` field, initially undefined when a `MyPlan` is created, that is used for caching the
+  inverse plan.
 
-* Define a new method `AbstractFFTs.plan_fft(x, region; kws...)` that returns a `MyPlan` for at least some types of `x` and some set of dimensions `region`. 
+* Define a new method `AbstractFFTs.plan_fft(x, region; kws...)` that returns a `MyPlan` for at least some types of
+  `x` and some set of dimensions `region`.
 
 * Define a method of `A_mul_B!(y, p::MyPlan, x)` that computes the transform `p` of `x` and stores the result in `y`.
 
-* If the inverse transform is implemented, you should also define `plan_inv(p::MyPlan)`, which should construct the inverse plan to `p`, and `plan_bfft(x, region; kws...)` for an unnormalized inverse ("backwards") transform of `x`. 
+* Define a method of `*(p::MyPlan, x)`, which can simply call your `A_mul_B!` method.
+  This is not defined generically in this package due to subtleties that arise for in-place and real-input FFTs.
+
+* If the inverse transform is implemented, you should also define `plan_inv(p::MyPlan)`, which should construct the
+  inverse plan to `p`, and `plan_bfft(x, region; kws...)` for an unnormalized inverse ("backwards") transform of `x`.
 
 * You can also define similar methods of `plan_rfft` and `plan_brfft` for real-input FFTs.
 
-The normalization convention for your FFT should be that it computes yₖ = ∑ⱼ xⱼ exp(-2πi jk/n) for a transform of length n, and the "backwards" (unnormalized inverse) transform computes the same thing but with exp(+2πi jk/n).
+The normalization convention for your FFT should be that it computes yₖ = ∑ⱼ xⱼ exp(-2πi jk/n) for a transform of
+length n, and the "backwards" (unnormalized inverse) transform computes the same thing but with exp(+2πi jk/n).