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.

Author: Kixunil

README.md

@@ -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