message: Document threading support a bit more
  This adds a few known limitations around threading to the README and suggests the users to look into GitHub issues to learn more.
diff: @@ -59,6 +59,11 @@ in your program, and cannot run all programs:
  has no access to most platform-specific APIs or FFI. A few APIs have been
  implemented (such as printing to stdout) but most have not: for example, Miri
  currently does not support SIMD or networking.
  +* Threading support is not finished yet. E.g. weak memory effects are not
  + emulated and spin loops (without syscalls) just loop forever. There's no
  + threading support on Windows.
  +
  +Consider looking into GitHub isues for more information about the limitations.
  [rust]: https://www.rust-lang.org/
  [mir]: https://github.com/rust-lang/rfcs/blob/master/text/1211-mir.md
  