Add README.md.

README.md

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+# tabfs
+
+## Setup
+
+You need to both install the Chrome extension and run the native
+filesystem.
+
+### Install the Chrome extension
+
+Go to the [Chrome extensions page](chrome://extensions).
+
+Enable Developer mode. Load-unpacked the `extension/` folder in this repo.
+
+### Run the C filesystem
+
+First, make sure you `git submodule update --init` to get the `mmx`
+and `cJSON` dependencies. And make sure you have FUSE.
+
+```
+$ cd fs
+$ mkdir mnt
+$ make [unmount] mount
+```
+
+### Connect the browser extension to the filesystem
+
+Once the filesystem is running and awaiting a WebSocket connection,
+you need to tell the browser extension to connect to it.
+
+Click the 'T' icon the extension put in your browser toolbar. The icon
+badge should change from red to blue, and the filesystem program
+should print that it's connected in the terminal.
+
+Now your browser tabs should be mounted in `fs/mnt`!
+
+## Design
+
+- `extension/`: Browser extension, written in JS
+- `fs/`: Native FUSE filesystem, written in C
+  - `tabfs.c`: Main thread. Talks to FUSE, implements fs operations.
+  - `ws.c`: Side thread. Runs WebSocket server. Talks to browser.
+  - `common.c`: Communications interface between tabfs and ws.
+
+When you, say, `cat` a file in the tab filesystem:
+
+1. `cat` makes something like a `read` syscall,
+
+2. which goes to the FUSE kernel module which backs that filesystem,
+
+3. FUSE forwards it to the `tabfs_read` implementation in our
+   userspace filesystem in `fs/tabfs.c`,
+
+4. then `tabfs_read` rephrases the request as a JSON string and
+   forwards it using `common_send_tabfs_to_ws` to `fs/ws.c`,
+
+5. and `fs/ws.c` forwards it to our browser extension over WebSocket
+   connection;
+
+6. our browser extension in `extension/background.js` handles the
+   incoming message and calls the browser APIs to construct the data
+   for that synthetic file;
+
+7. then the data gets sent back in a JSON message to `ws.c` and then
+   back to `tabfs.c` and finally back to FUSE and the kernel and
+   `cat`.
+
+(very little actual work happened here, tbh. it's all just
+marshalling)
+
+TODO: make diagrams?