message: Update usage docs in README
diff: @@ -20,21 +20,35 @@ The reference interoperable Golang implementation is available at
  ## Usage
  ```
  -Usage: rage [OPTIONS] [INPUT]
  +Usage:
  + rage -r RECIPIENT [-a] [-o OUTPUT] [INPUT]
  + rage --decrypt [-i IDENTITY] [-o OUTPUT] [INPUT]
  Positional arguments:
  - INPUT file to read input from (default stdin)
  + INPUT Path to a file to read from.
  Optional arguments:
  - -h, --help print help message
  - -V, --version print version info and exit
  - -d, --decrypt decrypt the input (default is to encrypt)
  - -p, --passphrase use a passphrase instead of public keys
  - --max-work-factor WF maximum work factor to allow for passphrase decryption
  - -a, --armor create ASCII armored output (default is age binary format)
  - -r, --recipient RECIPIENT recipient to encrypt to (may be repeated)
  - -i, --identity IDENTITY identity to decrypt with (may be repeated)
  - -o, --output OUTPUT output to OUTPUT (default stdout)
  + -h, --help Print this help message and exit.
  + -V, --version Print version info and exit.
  + -d, --decrypt Decrypt the input.
  + -p, --passphrase Encrypt with a passphrase instead of recipients.
  + --max-work-factor WF Maximum work factor to allow for passphrase decryption.
  + -a, --armor Encrypt to a PEM encoded format.
  + -r, --recipient RECIPIENT Encrypt to the specified RECIPIENT. May be repeated.
  + -i, --identity IDENTITY Use the private key file at IDENTITY. May be repeated.
  + -o, --output OUTPUT Write the result to the file at path OUTPUT.
  +
  +INPUT defaults to standard input, and OUTPUT defaults to standard output.
  +
  +RECIPIENT can be:
  +- An age public key, as generated by rage-keygen ("age1...").
  +- An SSH public key ("ssh-ed25519 AAAA...", "ssh-rsa AAAA...").
  +- A path or HTTPS URL to a file containing age recipients, one per line
  + (ignoring "#" prefixed comments and empty lines).
  +
  +IDENTITY is a path to a file with age identities, one per line
  +(ignoring "#" prefixed comments and empty lines), or to an SSH key file.
  +Multiple identities may be provided, and any unused ones will be ignored.
  ```
  ### Multiple recipients
  @@ -57,7 +71,7 @@ $ rage -p -o example.png.age example.png
  Type passphrase (leave empty to autogenerate a secure one): [hidden]
  Using an autogenerated passphrase:
  kiwi-general-undo-bubble-dwarf-dizzy-fame-side-sunset-sibling
  -$ rage -d -p example.png.age >example.png
  +$ rage -d example.png.age >example.png
  Type passphrase: [hidden]
  ```
  