Surface ImplementationStatus.md more publicly (#113)

Author: ngzhian

README.md

@@ -12,6 +12,8 @@ resulted.
 
 The [proposed specification](proposals/simd/SIMD.md) has the details.
 
+Note: this proposal is still being worked out, and rate of changes is high, please consult the [implementation status document](proposals/simd/ImplementationStatus.md) to get an idea of the state of implementation across toolchains and embedders.
+
 [Design issue](https://github.com/WebAssembly/proposals/issues/1)