message: Update README.md
  Closes
diff: ## Disclaimers
  * This library does not provide constant-time guarantees.
  -* This library relies on Rust's `i128_type` feature, which is currently only available in the nightly compiler.
  ## Usage
  @@ -13,10 +12,10 @@ Add the `ff` crate to your `Cargo.toml`:
  ```toml
  [dependencies]
  -ff = "0.2"
  +ff = "0.3"
  ```
  -The `ff` crate contains `Field`, `PrimeField`, `PrimeFieldRepr` and `SqrtField` traits. See the **[documentation](https://docs.rs/ff/0.2.0/ff/)** for more.
  +The `ff` crate contains `Field`, `PrimeField`, `PrimeFieldRepr` and `SqrtField` traits. See the **[documentation](https://docs.rs/ff/0.3.0/ff/)** for more.
  ### #![derive(PrimeField)]
  