Update README with a tutorial on passworded gpg keys. Add --batch to gpg  decryption to support it.

Hi, Daniel,

here's support of my workaround of how I use passworded gpg keys with wal-e successfully.

If the private key file is passworded, the recovery command will always fail without "--batch", even when the password is present in gpg agent.

This is because gpg always requires a tty, if --batch is not specified, and there's not way to access tty in the postgres restore_command.