update readme

Author: Hidenori MATSUKI

README.md

@@ -1,3 +1,31 @@
 # OpenSSH AuthorizedKeysCommand GitHub
 
 ![GitHub Actions status for "test-all"](https://github.com/mazgi/openssh-authorizedkeyscommand-github/workflows/test-all/badge.svg)
+
+Get your SSH Public Key from GitHub.
+
+```console
+> openssh-authorizedkeyscommand-github --help
+OpenSSH AuthorizedKeysCommand GitHub
+
+Usage:
+  openssh-authorizedkeyscommand-github --user=<user>
+  openssh-authorizedkeyscommand-github (--help | --version)
+
+Options:
+  --user=<user>   GitHub user.
+  -V, --version   Show version.
+  -h, --help      Show this screen.
+```
+
+## How to use
+
+1. Place the binary in your `$PATH`.
+
+2. Update `/etc/ssh/sshd_config`.
+
+```ssh_config
+AuthorizedKeysCommand /usr/local/bin/openssh-authorizedkeyscommand-github --user %u
+```
+
+3. Restart SSH server.