Add a README with the CLI usage

README.md

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+Elephantine
+---
+
+A pinentry program that allows shelling out to an arbitrary program to get user input.
+We aim to implement enough of the Assuan protocol to support this use case.
+
+# Usage
+```
+Implements the pinentry protocol and uses walker for PIN input
+
+Usage: elephantine [OPTIONS]
+
+Options:
+  -d, --debug...                       The debug level [env: ELEPHANTINE_DEBUG=]
+      --config-file <FILE>             Path to the configuration file [env: ELEPHANTINE_CONFIG_FILE=] [default: /home/narthana/.config/elephantine/elephantine.toml]
+  -D, --display [<DISPLAY>]            The X display to use for the dialog [env: PINENTRY_DISPLAY=]
+  -T, --ttyname [<FILE>]               The tty terminal node name [env: TTYNAME=]
+  -N, --ttytype [<NAME>]               [env: TTYTYPE=]
+  -C, --lc-ctype [<STRING>]            The `LC_CTYPE` locale category [env: LC_CTYPE=]
+  -M, --lc-messages [<STRING>]         The `LC_MESSAGES` value [env: LC_MESSAGES=]
+  -o, --timeout [<SECS>]               Timeout in seconds for requests that show dialogs to the user. E.g. GETPIN, CONFIRM, etc [env: ELEPHANTINE_TIMEOUT=] [default: 300]
+  -g, --no-local-grab <NO_LOCAL_GRAB>  Grab keyboard only while the window is focused [env: ELEPHANTINE_NO_LOCAL_GRAB=] [possible values: true, false]
+  -W, --parent-wid [<WINDOW_ID>]       Parent window ID (for partitioning)
+  -c, --colors [<STRING>]              Custom colors for the dialog
+  -a, --ttyalert [<STRING>]            The alert mode (none, beep, or flash)
+      --command <COMMAND>...           The command to run the dialog. It must print the input to stdout [default: "walker --password"]
+  -h, --help                           Print help
+  -V, --version                        Print version
+```