Add a Debugging section to the README.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 ## Writing Tests
 
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 
 [test]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test
 [expect]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_expect
 
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 Both the annotation and the parameter take a map. The map's keys are [platform
 selectors](#platform-selector-syntax) which describe the platforms for which the
 specialized configuration applies. Its values are instances of some of the same
 annotation classes that can be used for a suite: `Skip` and `Timeout`. A value
 can also be a list of these values.
 
 If multiple platforms match, the configuration is applied in order from first to
 last, just as they would in nested groups. This means that for configuration
 like duration-based timeouts, the last matching value wins.
 
+## Debugging
+
+Tests can be debugged interactively using browsers' built-in development tools,
+including Observatory when you're using Dartium. Currently there's no support
+for interactively debugging command-line VM tests, but it will be added
+[in the future][issue 50].
+
+[issue 50]: https://github.com/dart-lang/test/issues/50
+
+The first step when debugging is to pass the `--pause-after-load` flag to the
+test runner. This pauses the browser after each test suite has loaded, so that
+you have time to open the development tools and set breakpoints. For Dartium,
+the test runner will print the Observatory URL for you. For PhantomJS, it will
+print the remote debugger URL. For content shell, it'll print both!
+
+Once you've set breakpoints, either click the big arrow in the middle of the web
+page or press Enter in your terminal to start the tests running. When you hit a
+breakpoint, the runner will open its own debugging console in the terminal that
+controls how tests are run. You can type "restart" there to re-run your test as
+many times as you need to figure out what's going on.
+
 ## Testing With `barback`
 
 Packages using the `barback` transformer system may need to test code that's
 created or modified using transformers. The test runner handles this using the
 `--pub-serve` option, which tells it to load the test code from a `pub serve`
 instance rather than from the filesystem.
 
 Before using the `--pub-serve` option, add the `test/pub_serve` transformer to
 your `pubspec.yaml`. This transformer adds the necessary bootstrapping code that
 allows the test runner to load your tests properly:
@@ skipping 38 matching lines @@
 "pub serve" is compiling test/utils_test.dart...
 00:00 +42: All tests passed!
 ```
 
 ## Further Reading
 
 Check out the [API docs][api] for detailed information about all the functions
 available to tests.
 
 [api]: http://www.dartdocs.org/documentation/test/latest/index.html