Updated README to include 'list' example and usage

Author: iamvery

README.md

@@ -25,6 +25,9 @@ with:
 ```bash
 $ gh-auth add chrishunt
 Adding 2 key(s) to '/Users/chris/.ssh/authorized_keys'
+
+$ gh-auth list
+Added users: chrishunt, zachmargolis
 ```
 
 That was easy! When we're done working, you can revoke my access with:
@@ -72,7 +75,7 @@ Removing 2 key(s) from '/Users/chris/.ssh/authorized_keys'
 `gh-auth` can be used from the command line after the gem has been installed.
 
 ```bash
-usage: gh-auth [--version] [add|remove] <username>
+usage: gh-auth [--version] [add|remove|list] <username>
 ```
 
 ### In Your Project
@@ -102,7 +105,7 @@ Install the `github-auth` gem:
 $ gem install github-auth
 
 $ gh-auth
-usage: gh-auth [--version] [add|remove] <username>
+usage: gh-auth [--version] [add|remove|list] <username>
 ```
 
 ### SSH Public Key Authentication (Mac OS X)