Place ff_derive re-exports behind a feature

Part of #3.

README.md

@@ -21,6 +21,15 @@ The `ff` crate contains `Field`, `PrimeField`, `PrimeFieldRepr` and `SqrtField`
 
 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.
 
+First, enable the `derive` crate feature:
+
+```toml
+[dependencies]
+ff = { version = "0.3", features = ["derive"] }
+```
+
+And then use the macro like so:
+
 ```rust
 extern crate rand;
 #[macro_use]