cmd/age: add -help and README

cmd/age/age.go

@@ -27,8 +27,36 @@ func (f *multiFlag) Set(value string) error {
 	return nil
 }
 
+const usage = `Usage:
+    age -r RECIPIENT [-a] [-o OUTPUT] [INPUT]
+    age --decrypt [-i KEY] [-o OUTPUT] [INPUT]
+
+Options:
+    -o OUTPUT                   Write the result to the file at path OUTPUT.
+    -a, --armor                 Encrypt to a PEM encoded format.
+    -p, --passphrase            Encrypt with a passphrase.
+    -r, --recipient RECIPIENT   Encrypt to the specified RECIPIENT. Can be repeated.
+    -d, --decrypt               Decrypt the input to the output.
+    -i, --identity KEY          Use the private key file at path KEY. Can be repeated.
+
+INPUT defaults to standard input, and OUTPUT defaults to standard output.
+
+RECIPIENT can be an age public key, as generated by age-keygen, ("age1...")
+or an SSH public key ("ssh-ed25519 AAAA...", "ssh-rsa AAAA...").
+
+KEY is a path to a file with age secret keys, one per line
+(ignoring "#" prefixed comments and empty lines), or to an SSH key file.
+Multiple keys can be provided, and any unused ones will be ignored.
+
+Example:
+    $ age-keygen -o key.txt
+    Public key: age1ql3z7hjy54pw3hyww5ayyfg7zqgvc7w3j2elw8zmrj2kg5sfn9aqmcac8p
+    $ tar cvz ~/data | age -r age1ql3z7hjy54pw3hyww5ayyfg7zqgvc7w3j2elw8zmrj2kg5sfn9aqmcac8p > data.tar.gz.age
+    $ age -d -i key.txt -o data.tar.gz data.tar.gz.age`
+
 func main() {
 	_log.SetFlags(0)
+	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
 
 	var (
 		outFlag                          string