Create reference.md

Moved some stuff from original README in here; still needs some editing

docs/reference.md

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+# Reference Manual: disko
+
+## Module Options
+
+TODO: Still to be documented
+
+## # Reference Manual: disko
+
+## Command Line Options
+
+```
+$ nix run github:nix-community/disko --
+disko [options] disk-config.nix
+or disko [options] --flake github:somebody/somewhere
+Options:
+* -m, --mode mode
+  set the mode, either create or mount
+* -f, --flake uri
+  fetch the disko config relative to this flake's root
+* --arg name value
+  pass value to nix-build. can be used to set disk-names for example
+* --argstr name value
+  pass value to nix-build as string
+* --root-mountpoint /mnt
+  where to mount the device tree
+* --dry-run
+  just show the path to the script instead of running it
+* --debug
+  run with set -x
+```
+
+##