Place ff_derive re-exports behind a feature

Part of #3.

Cargo.toml

@@ -14,6 +14,6 @@ rand = "0.4"
 ff_derive = { version = "0.3.0", path = "ff_derive", optional = true }
 
 [features]
-default = ["derive"]
+default = []
 u128-support = []
 derive = ["ff_derive"]

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]

src/lib.rs

@@ -3,9 +3,11 @@
 extern crate byteorder;
 extern crate rand;
 
+#[cfg(feature = "derive")]
 #[macro_use]
 extern crate ff_derive;
 
+#[cfg(feature = "derive")]
 pub use ff_derive::*;
 
 use std::error::Error;