Update guides/source/contributing_to_ruby_on_rails.md

Co-authored-by: Eileen M. Uchitelle <[email protected]>

Author: AlisonAcuna

guides/source/contributing_to_ruby_on_rails.md

@@ -482,6 +482,10 @@ $ bundle exec rake TEST=test/cases/associations/has_many_associations_test.rb
 
 You can invoke `test_jdbcmysql`, `test_jdbcsqlite3` or `test_jdbcpostgresql` also. See the file `activerecord/RUNNING_UNIT_TESTS.rdoc` for information on running more targeted database tests.
 
+#### Using Debuggers with Test
+
+To use an external debugger (pry, byebug, etc), install the debugger and use it as normal.  If debugger issues occur, run tests in serial by setting `PARALLEL_WORKERS=1` or run a single test with `-n test_long_test_name `.
+
 ### Warnings
 
 The test suite runs with warnings enabled. Ideally, Ruby on Rails should issue no warnings, but there may be a few, as well as some from third-party libraries. Please ignore (or fix!) them, if any, and submit patches that do not issue new warnings.