Merge pull request #21184 from brooksreese/touch_on_unit_testing

[ci skip] Add link to testing guide

guides/source/command_line.md

@@ -260,7 +260,13 @@ $ bin/rake db:migrate
 ==  CreateHighScores: migrated (0.0019s) ======================================
 ```
 
-INFO: Let's talk about unit tests. Unit tests are code that tests and makes assertions about code. In unit testing, we take a little part of code, say a method of a model, and test its inputs and outputs. Unit tests are your friend. The sooner you make peace with the fact that your quality of life will drastically increase when you unit test your code, the better. Seriously. We'll make one in a moment.
+INFO: Let's talk about unit tests. Unit tests are code that tests and makes assertions 
+about code. In unit testing, we take a little part of code, say a method of a model, 
+and test its inputs and outputs. Unit tests are your friend. The sooner you make 
+peace with the fact that your quality of life will drastically increase when you unit 
+test your code, the better. Seriously. Please visit 
+[the testing guide](http://guides.rubyonrails.org/testing.html) for an in-depth 
+look at unit testing.
 
 Let's see the interface Rails created for us.