message: doc: note nightly version
  I don't really know how this is supposed to help. Nobody tracks
  arbitrarily old nightly versions of the Rust compiler, but at least one
  person has requested this.
  Closes
diff: @@ -192,6 +192,8 @@ assert!(matches.matched(6));
  ### Usage: enable SIMD optimizations
  SIMD optimizations are enabled automatically on Rust stable 1.27 and newer.
  +For nightly versions of Rust, this requires a recent version with the SIMD
  +features stabilized.
  ### Usage: a regular expression parser
  