Recommend installing a new version of wasm-opt. (#3452)

Update contributing guidelines to include a recommendation for installing `wasm-opt`.

# Important Notes
[ci no changelog needed]

app/gui/docs/CONTRIBUTING.md

@@ -102,6 +102,19 @@ setup:
   environment variable to explicitly enable regeneration of the interface files.
   The `flatc` is run as part of `build.rs` script of the `enso-protocol package.
 
+- **(Optional) The Latest version of `wasm-opt`**
+
+  The version of `wasm-opt` that comes bundled with `wasm-pack` can be out of
+  date and slow. We recommend to install a newer version from the
+  [binaryen releases](https://github.com/WebAssembly/binaryen/releases), or
+  [compile the newest version from scratch](https://github.com/WebAssembly/binaryen#building=).
+  `wasm-pack` will pick up the locally installed version automatically and use
+  it instead of the bundled one. Binaryen packages are also available through
+  some system package managers (apt/pacman/brew), check whether the version
+  there is newer than the bundled version of wasm-pack (which is true for all of
+  them as of wasm-pack 0.10.2, which bundles version 90 which was released Dec
+  2019).
+
 <br/>
 
 ## Working with sources