Add readme

README.md

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+cache-fs
+--------
+
+caching fs to use over immutable network filesystems
+
+The use-case is something like your Steam Deck has a large directory of roms mounted over NFS, but if you disable wifi,
+you want the roms you've already played still available.
+
+This basically bind-mounts your NFS share over a path of your choosing, except it caches all file attributes/paths
+forever, and copies the files you open to your cache directory. When you access that file again it doesn't access the
+remote server at all.  So if your remote (say NFS) server is not available, you can still play the games.
+
+If you add/change files to your remote, you must delete `/local/cache/dir/cache-fs.tree` and remount.
+
+Usage
+-----
+
+This assumes you install the `cache-fs` binary as `mount.cachefs`, then you can use it in /etc/fstab etc
+```
+mount -t cachefs -o cache_dir=/local/cache/dir /remote/dir/to/cache /where/you/want/it/mounted
+```