Added more granularity to the README.

After using this gem to implement multiple mail APIs on the fly I found the documentation to be a bit difficult to follow at times. While the documentation mentions the tight integration with the Mail gem, it assumes a lot of knowledge about how this gem works. By changing the README to include full examples for working with the Mail gem, rather than partial examples simply with delivery method, multi_mail should be even easier for new users (especially those without domain knowledge of the Mail gem) to get up and running with multi_mail and whatever mail services they are currently using.