Add RUNNING_UNIT_TESTS to ActionMailer [ci skip]

ActionPack, ActionRecord and ActionView folders have their own
`RUNNING_UNIT_TESTS.rdoc` file. ActionMailer should too… right?

In my opinion, it makes sense, and it could also be added to the
other folders that are missing it (ActiveJob, ActiveSupport, Railties)
but I'd like to know your opinion first. Thanks!