Add documentation for sending region values to Mandrill

I have not found documentation about how to send values for template regions. After a little research I know how to do it: adding the data in the 'mandrill_template_content' key of the $message varaible in a hook_mail_alter().

I think this information should be added to the module in the README.txt.

I propose add this paragraph:

To send values from Drupal to Mandrill that will be used to substitute the
template regions you should implement hook_mail_alter() and add your values in
the 'mandrill_template_content' key in the $message array as an array with two
keys: name and content.

For example, for sending the value 'foo value' for the region 'foo' you
can use this code:

/**
* Implements hook_mail_alter();
*/
function mymodule_mail_alter(&$message) {
$message['mandrill_template_content'][] = array(
'name' => 'foo',
'content' => 'foo value',
);
}