message: Add "Features" section to crate-level docs
  An especially good idea, since random number generation is behind a
  feature, and the example in the crate-level docs does random number
  generation.
diff: //! # }
  //! ```
  //!
  +//! See the "Features" section for instructions for enabling random number generation.
  +//!
  +//! ## Features
  +//!
  +//! The `std` crate feature is mandatory and enabled by default. If you depend on
  +//! `num-bigint` with `default-features = false`, you must manually enable the
  +//! `std` feature yourself. In the future, we hope to support `#![no_std]` with
  +//! the `alloc` crate when `std` is not enabled.
  +//!
  +//! Implementations for `i128` and `u128` are only available with Rust 1.26 and
  +//! later. The build script automatically detects this, but you can make it
  +//! mandatory by enabling the `i128` crate feature.
  +//!
  +//! ### Random Generation
  +//!
  +//! `num-bigint` supports the generation of random big integers when the `rand`
  +//! feature is enabled. To enable it include rand as
  +//!
  +//! ```toml
  +//! rand = "0.5"
  +//! num-bigint = { version = "0.2", features = ["rand"] }
  +//! ```
  +//!
  +//! Note that you must use the version of `rand` that `num-bigint` is compatible
  +//! with: `0.5`.
  +//!
  +//!
  //! ## Compatibility
  //!
  //! The `num-bigint` crate is tested for rustc 1.15 and greater.
  