Add minitest-spec-rails to guide's testing options

guides/source/testing.md

@@ -977,5 +977,6 @@ The built-in `test/unit` based testing is not the only way to test Rails applica
 * [NullDB](http://avdi.org/projects/nulldb/), a way to speed up testing by avoiding database use.
 * [Factory Girl](https://github.com/thoughtbot/factory_girl/tree/master), a replacement for fixtures.
 * [Machinist](https://github.com/notahat/machinist/tree/master), another replacement for fixtures.
+* [MiniTest::Spec Rails](https://github.com/metaskills/minitest-spec-rails), use the MiniTest::Spec DSL within your rails tests.
 * [Shoulda](http://www.thoughtbot.com/projects/shoulda), an extension to `test/unit` with additional helpers, macros, and assertions.
 * [RSpec](http://relishapp.com/rspec), a behavior-driven development framework