[WebAssembly] Update release notes

Summary:
Explicitly note that multithreading support is not included in the stable
ABI.

Differential Revision: https://reviews.llvm.org/D56681

llvm-svn: 351213
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index 3ffebf1..db9cf51 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -105,6 +105,8 @@
 the object file format has an ABI versioning capability, and one anticipated
 use for it will be to add support for returning small structs as multiple
 return values, once the underlying WebAssembly platform itself supports it.
+Additionally, multithreading support is not yet included in the stable ABI.
+
 
 Changes to the OCaml bindings
 -----------------------------