Add docs for podman specific compose file extensions

Signed-off-by: GnSight <[email protected]>

docs/Extensions.md

@@ -0,0 +1,23 @@
+# Overview
+
+Podman-compose extends the compose specification to support some unique features of Podman. These extensions can be specified in the compose file under the "x-podman" field.
+
+Currently, podman-compose supports the following extensions:
+
+* `uidmap` - Run the container in a new user namespace using the supplied UID mapping.
+
+* `gidmap` - Run the container in a new user namespace using the supplied GID mapping.
+
+* `rootfs` - Run the container without requiring any image management; the rootfs of the container is assumed to be managed externally.
+
+For example, the following docker-compose.yml allows running a podman container with externally managed rootfs.
+```yml
+version: "3"
+services:
+    my_service:
+      command: ["/bin/busybox"]
+      x-podman:
+        rootfs: "/path/to/rootfs"
+```
+
+For explanations of these extensions, please refer to the [Podman Documentation](https://docs.podman.io/).