note region in README for devs

Author: stevengj

README.md

@@ -23,7 +23,7 @@ To define a new FFT implementation in your own module, you should
   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`.
+  `x` and some set of dimensions `region`.   The `region` (or a copy thereof) should be accessible via `region(p::MyPlan)` (which defaults to `p.region`).
 
 * Define a method of `LinearAlgebra.mul!(y, p::MyPlan, x)` (or `A_mul_B!(y, p::MyPlan, x)` on Julia prior to
   0.7.0-DEV.3204) that computes the transform `p` of `x` and stores the result in `y`.