add 'tty' usage

README.md

@@ -1,16 +1,14 @@
 # getpass - a Go module to get a password
 
-The `getpass` module provides a simple way to retrieve
-a password from the user by specifying a number of
-different password sources:
+The `getpass` module provides a simple way to retrieve a password from
+the user by specifying a number of different password sources:
 
 ```
 func Getpass(passfrom, prompt string) (pass string, err error)
 ```
 
-Getpass retrieves a password from the user using a
-method defined by the `passfrom` string.  The
-following methods are supported:
+Getpass retrieves a password from the user using a method defined by
+the `passfrom` string.  The following methods are supported:
 
 `env:var` -- Obtain the password from the environment variable var.
 Since the environment of other processes may be visible
@@ -26,23 +24,22 @@ controls should be used to protect this file.
 password from the macOS keychain.
 
 `lpass:name` -- Use the LastPass command-line client lpass(1) to
-retrieve the named password.  You should previously have
-run 'lpass login' for this to work.
+retrieve the named password.  You should previously have run 'lpass
+login' for this to work.
 
-`op:name` -- Use the 1Password command-line client op(1) to
-retrieve the named password.
+`op:name` -- Use the 1Password command-line client op(1) to retrieve
+the named password.
 
-`pass:password` -- The actual password is password.  Since the password is
-visible to utilities such as ps(1) and possibly leaked
-into the shell history file, this form should only be
-used where security is not important.
+`pass:password` -- The actual password is password.  Since the
+password is visible to utilities such as ps(1) and possibly leaked
+into the shell history file, this form should only be used where
+security is not important.
 
-If no password retrieval method is specified, then
-`Getpass` will prompt the user on the controlling tty
-using the provided `prompt`.
+`tty` -- This is the default if no `passfrom` method is specified:
+`Getpass` `Getpass` will prompt the user on the controlling tty using
+the provided `prompt`.
 
-If no `prompt` is provided, then `Getpass` will use
-"Password: ".
+If no `prompt` is provided, then `Getpass` will use "Password: ".
 
 ## Examples