message: doc: note the new `unstable` feature
diff: @@ -188,6 +188,16 @@ assert!(!matches.matched(5));
  assert!(matches.matched(6));
  ```
  +### Usage: enable SIMD optimizations
  +
  +This crate provides an `unstable` feature that can only be enabled on nightly
  +Rust. When this feature is enabled, the regex crate will use SIMD optimizations
  +if your CPU supports them. No additional compile time flags are required; the
  +regex crate will detect your CPU support at runtime.
  +
  +When `std::arch` becomes stable, then these optimizations will be enabled
  +automatically.
  +
  ### Usage: a regular expression parser
  