add README for cuda

cuda/README

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+Before you start this compose, you shoud do:
+
+1. Check the volumes to be mounted.
+2. Ensure the nvidia-container-toolkit already installed, this
+    is the name on AUR for ArchLinux.
+3. If you mount /root, make sure it's owned by root and with
+    750 or less permission, this is required by sshd, or it will
+    not accept any authorized key.
+
+Then to build the image:
+
+    podman-compose build
+
+Then first start:
+
+    podman-compose up --detach
+
+To stop:
+
+    podman stop mcuda
+
+To re-start:
+
+    podman start mcuda