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
+```