Added docs for Mandrill Service and fixed typo in README

Noticed docs for Mandrill Service were missing so I added them, wanted to clarify about this

" * Options:
 * 
 * - mandrill
 *   Initialised Mandrill API instance
 * 
 * - tags
 *   Array of tags to send to Mandrill
 * 
"

Should I mention something about the tags options?