Add flexible vectors to 2020-04-28

After SIMD sync call (WebAssembly/simd#202) it looks like we are ready to move the long vector idea a bit further.

Author: penzn

2020/CG-04-28.md

@@ -29,6 +29,7 @@ Installation is required, see the calendar invite.
        See https://github.com/WebAssembly/wasm-c-api/issues/132 for background.
        Poll: Should wasm-c-api pursue safety, as discussed in this issue?
        Poll: If not, approve a new WebAssembly C API proposal at phase 0?
+    1. Flexible vectors - [slides](https://github.com/penzn/wasm-long-vectors), product of discussion in [simd/29](https://github.com/WebAssembly/simd/issues/29). Poll: Approve phase 0 for flexible vectors.
 1. Closure
 
 ## Agenda items for future meetings