README.md: document --dir (#338)

README.md

@@ -274,6 +274,50 @@ tart clone acme.io/remoteorg/name:latest my-local-vm-name
 
 This invocation calls the `tart pull` implicitly (if the image is not being present) before doing the actual cloning.
 
+### Mounting directories
+
+To mount a directory, run the VM with the `--dir` argument:
+
+```sh
+tart run --dir=project:~/src/project vm
+```
+
+Here, the `project` specifies a mount name, whereas the `~/src/project` is a path to the host's directory to expose to the VM.
+
+It is also possible to mount directories in read-only mode by adding a third parameter, `ro`:
+
+```sh
+tart run --dir=project:~/src/project:ro vm
+```
+
+To mount multiple directories, repeat the `--dir` argument for each directory:
+
+```sh
+tart run --dir=www1:~/project1/www --dir=www2:~/project2/www
+```
+
+Note that the first parameter in each `--dir` argument must be unique, otherwise only the last `--dir` argument using that name will be used.
+
+Note: to use the directory mounting feature, the host needs to run macOS 13.0 (Ventura) or newer.
+
+#### Accessing mounted directories in macOS guests
+
+All shared directories are automatically mounted to `/Volumes/My Shared Files` directory.
+
+The directory we've mounted above will be accessible from the `/Volumes/My Shared Files/project` path inside a guest VM.
+
+Note: to use the directory mounting feature, the guest VM needs to run macOS 13.0 (Ventura) or newer.
+
+#### Accessing mounted directories in Linux guests
+
+To be able to access the shared directories from the Linux guest, you need to manually mount the virtual filesystem first:
+
+```sh
+mount -t virtiofs com.apple.virtio-fs.automount /mnt/shared
+```
+
+The directory we've mounted above will be accessible from the `/mnt/shared/project` path inside a guest VM.
+
 ## FAQ
 
 <details>