Add passthrough backend to readme

Author: james-pre

readme.md

@@ -10,6 +10,7 @@ ZenFS is modular and extensible. The core includes some built-in backends:
 -   `Overlay`: Use a read-only file system as read-write by overlaying a writable file system on top of it. ([copy-on-write](https://en.wikipedia.org/wiki/Copy-on-write))
 -   `Fetch`: Downloads files over HTTP with the `fetch` API (_readonly_)
 -   `Port`: Interacts with a remote over a `MessagePort`-like interface (e.g. a worker)
+-   `Passthrough`: Use an existing `node:fs` interface with ZenFS
 
 ZenFS supports a number of other backends. Many are provided as separate packages under `@zenfs`. More backends can be defined by separate libraries by extending the `FileSystem` class and providing a `Backend` object.