Clarify that by default the gem_name is rails

[ignacio-chiazzo]

I am using this gem on a repository that doesn't use Rails. I migrated from ActiveSupport::Deprecation.warn(...) to ActiveSupport::Deprecation.new.warn(), and it did not log any deprecation calls.

The reason is that, by default, the gem attaches to :rails.

This PR clarifies in the README that if you're not using Rails, you should set attach_to correctly.

Example:

DeprecationToolkit::Configuration.configure do |config|
  config.attach_to = ["my_cool_gem", "test_case"]
  config.behaviour = :notify
  # more stuff
end