[WebAssembly] Update release notes

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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351213 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed