docs: update README.browser in fixtures

Author: topheman

fixtures/filesystem/README.browser.md

@@ -1 +1,11 @@
 # filesystem
+
+You are interacting with a virtual filesystem, in your browser!
+
+The plugins `ls` and `cat` do real filesystem operations in their source code, like `std::fs::read_dir` or `std::fs::read_to_string`.
+
+Those operations are forwarded via the `@bytecodealliance/preview2-shim/filesystem` shim, which shims the `wasi:filesystem` filesystem interface.
+
+On the cli, you would interact with a real filesystem with the exact same source code as the one you are running (which was developed mainly in rust and compiled to WebAssembly, for the browser, using `jco`).
+
+The virtual filesystem was generated by the `prepareVirtualFs` command, which reads the `fixtures/filesystem` directory and generates a virtual filesystem in the [`packages/web-host/src/wasm/virtualFs.ts`](https://github.com/topheman/webassembly-component-model-experiments/blob/master/packages/web-host/src/wasm/virtualFs.ts) file.

packages/web-host/src/wasm/virtualFs.ts

@@ -18,7 +18,8 @@ export function makeVirtualFs() {
           "# This is a hidden file\n# It should only be visible when explicitly requested\n# Used for testing hidden file operations\n",
       },
       "README.md": {
-        source: "# filesystem\n",
+        source:
+          "# filesystem\n\nYou are interacting with a virtual filesystem, in your browser!\n\nThe plugins `ls` and `cat` do real filesystem operations in their source code, like `std::fs::read_dir` or `std::fs::read_to_string`.\n\nThose operations are forwarded via the `@bytecodealliance/preview2-shim/filesystem` shim, which shims the `wasi:filesystem` filesystem interface.\n\nOn the cli, you would interact with a real filesystem with the exact same source code as the one you are running (which was developed mainly in rust and compiled to WebAssembly, for the browser, using `jco`).\n\nThe virtual filesystem was generated by the `prepareVirtualFs` command, which reads the `fixtures/filesystem` directory and generates a virtual filesystem in the [`packages/web-host/src/wasm/virtualFs.ts`](https://github.com/topheman/webassembly-component-model-experiments/blob/master/packages/web-host/src/wasm/virtualFs.ts) file.\n",
       },
       data: {
         dir: {