chore: update migration guide TLA (#728)

docs/upgrade-to-v5.md

@@ -66,6 +66,17 @@ bin.value(true).toString('utf16le');
 new TextDecoder('utf-16le').decode(bin.value(true));
 ```
 
+### Bundling Top-Level Await in BSON
+
+The `"export"` conditions in [package.json](../package.json) make it possible for the `bson` package to provide module resolutions for ES Module and CommonJS.
+When using a bundler and targeting the browser the export condition `"browser"` will likely point your bundler at the bson.mjs file, an ES Module.
+This ES Module form of `BSON` works in both Node.js and browser environments, however, one sticking point is that bundlers often do not enable support of top-level await usage by default.
+
+For example, when using Webpack you may need to:
+- enable Webpack's [`topLevelAwait`](https://webpack.js.org/configuration/experiments/#experimentstoplevelawait) setting to bypass the issue.
+- override the [resolution](https://webpack.js.org/configuration/resolve/#resolve) of `bson` to use the CommonJS module, "bson.cjs".
+- `require('bson')` instead of using `import`.
+
 ## Other Changes
 
 ### TS "target" set to es2020