message: Modify README.
diff: @@ -20,7 +20,7 @@ The `ff` crate contains `Field`, `PrimeField`, `PrimeFieldRepr` and `SqrtField`
  ### #![derive(PrimeField)]
  -If you need an implementation of a prime field, this library also provides a procedural macro that will expand into an efficient implementation of a prime field when supplied with the modulus.
  +If you need an implementation of a prime field, this library also provides a procedural macro that will expand into an efficient implementation of a prime field when supplied with the modulus. `PrimeFieldGenerator` must be an element of Fp of p-1 order, that is also quadratic nonresidue.
  ```rust
  extern crate rand;
  @@ -28,7 +28,8 @@ extern crate rand;
  extern crate ff;
  #[derive(PrimeField)]
  -#[PrimeFieldModulus = "57896044618658097711785492504343953926634992332820282019728792003956564819949"]
  +#[PrimeFieldModulus = "52435875175126190479447740508185965837690552500527637822603658699938581184513"]
  +#[PrimeFieldGenerator = "7"]
  struct Fp(FpRepr);
  ```
  