Document ability to run a single test.

guides/source/development_dependencies_install.md

@@ -133,13 +133,20 @@ $ cd railties
 $ TEST_DIR=generators bundle exec rake test
 ```
 
-You can run any single test separately too:
+You can run the tests for a particular file by using:
 
 ```bash
 $ cd actionpack
 $ bundle exec ruby -Itest test/template/form_helper_test.rb
 ```
 
+Or, you can run a single test in a particular file:
+
+```bash
+$ cd actionpack
+$ bundle exec ruby -Itest path/to/test.rb -n test_name
+```
+
 ### Active Record Setup
 
 The test suite of Active Record attempts to run four times: once for SQLite3, once for each of the two MySQL gems (`mysql` and `mysql2`), and once for PostgreSQL. We are going to see now how to set up the environment for them.