Documentation (and abstract class?) for custom delivery method

[Mr0grog]

I was recently trying to build a custom delivery method for this gem and tripped over a few minor things while doing so, owing to the lack of documentation. It would be helpful if there was some documentation about this in the README and/or if there was an abstract base class for delivery methods to inherit from, e.g:

module Mail
  class AbstractDelivery
    attr_reader :settings

    def initialize(settings = {})
      @settings = settings
    end

    def deliver!(message)
      raise NotImplementedError, "`deliver!` method is not implemented on #{self.class.name}"
    end
  end
end

One of the things that surprised me, for example, was that I needed to expose a #settings attribute with the settings hash that was passed into the constructor.

(I was also surprised by the differing return values from Message#deliver and Message#deliver!, but that's not really an issue with the delivery method, and someone already posted a PR for that: #1313.)

I’m happy to write a PR for this; just let me know whether the abstract class is desirable in addition to docs.