Write usage info in README

README.md

+# Usage
+
+`cryptops-api --root-device=DEV --crypt-name=LABEL [--notify-command=COMMAND]`
+
+*   The `--root-device=DEV` option is mandatory. `DEV` should be the root device
+    of the machine, like `/dev/sda` or `/dev/xvda`.
+    
+    When the root device is still unencrypted (for example when starting
+    cryptops-api for the first time), this should be an unpartitioned device,
+    with the file system spanning the whole device.
+    
+    As part of the encryption process, this device will be partitioned and the
+    encrypted contents will be stored on the second partition, but DEV should
+    still point to the whole device, as before.
+
+*   The `--crypt-name=LABEL` option is mandatory. `LABEL` will be used by
+    cryptsetup to name the encrypted volume. This will be visible to the operating
+    system, as the decrypted root disk is available at `/dev/mapper/LABEL`.
+
+*   The `--notify-command=COMMAND` option is optional. The given `COMMAND` will be
+    run whenever cryptops-api detects that the root device needs the user to enter
+    their password to continue the booting process. You can use this to notify the
+    user of this situation.