Better Visibility for User Credentials Grant Type

[bshaffer]

From bshaffer/oauth2-server-php#459:

Hello,

Great work on this, very educational and helpful. Many thanks.

Here:
http://bshaffer.github.io/oauth2-server-php-docs/cookbook/

The server.php code should probably also include:
// Add the "password" grant type (this is where the oauth magic happens)
$server->addGrantType(new OAuth2\GrantType\UserCredentials($storage));

This is also important for people trying the demo app later.

Add a link to the demo app from the user credentials docs page, and add a link to the other grant types from the walkthrough page.