added information about TEST_DIR env variable

railties/guides/source/contributing_to_ruby_on_rails.textile

@@ -101,6 +101,14 @@ $ cd actionpack
 $ rake test
 </shell>
 
+If you want to run tests from the specific directory use the +TEST_DIR+ environment variable. For example, this will run tests inside +railties/test/generators+ directory only:
+
+<shell>
+$ cd railties
+$ TEST_DIR=generators rake test
+</shell>
+
+
 h4. Warnings
 
 The test suite runs with warnings enabled. Ideally Ruby on Rails should issue no warning, but there may be a few, and also some from third-party libraries. Please ignore (or fix!) them if any, and submit patches that do not issue new warnings.