Add a link to Binaryen's custom-page-sizes tracking issue

Author: fitzgen

proposals/custom-page-sizes/Overview.md

@@ -417,6 +417,7 @@ Engines:
 
 Toolchains:
 
+* [ ] [Binaryen and `wasm-opt`](https://github.com/WebAssembly/binaryen/issues/6873)
 * [ ] `wasm-ld` in LLVM
 * [x] [The Virgil language toolchain](https://github.com/titzer/virgil/pull/272)