doc(bundle-source): describe moduleFormats

packages/bundle-source/README.md

@@ -25,3 +25,34 @@ or inside an async function (and therefore outside of Jessie), do:
 const { moduleFormat, source, sourceMap } = await sourceBundleP;
 ...
 ```
+
+## getExport moduleFormat
+
+The first main `moduleFormat` is the `"getExport"` format.  It generates
+source like:
+
+```js
+function getExport() {
+  let exports = {};
+  const module = { exports };
+  // CommonJS source translated from the inputs.
+  ...
+  return module.exports;
+}
+```
+
+To evaluate it and obtain the resulting module namespace, you need to endow
+a `require` function to resolve external imports.
+
+## nestedEvaluate moduleFormat
+
+This is logically similar to the `getExport` format, except that the code
+may additionally depend upon a `nestedEvaluate(src)` function to be used
+to evaluate submodules in the same context as the parent function.
+
+The advantage of this format is that it helps preserve the filenames within
+the bundle in the event of any stack traces.
+
+Also, the toplevel `getExport(filePrefix = "/bundled-source")` accepts an
+optional `filePrefix` argument in order to help sanitize stack
+traces (which is prepended to relative paths for the bundled files).