Add basic support for a configuration file.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 * [Writing Tests](#writing-tests)
 * [Running Tests](#running-tests)
   * [Restricting Tests to Certain Platforms](#restricting-tests-to-certain-platforms)
   * [Platform Selector Syntax](#platform-selector-syntax)
   * [Running Tests on Dartium](#running-tests-on-dartium)
 * [Asynchronous Tests](#asynchronous-tests)
 * [Running Tests With Custom HTML](#running-tests-with-custom-html)
 * [Configuring Tests](#configuring-tests)
   * [Skipping Tests](#skipping-tests)
   * [Timeouts](#timeouts)
   * [Platform-Specific Configuration](#platform-specific-configuration)
+  * [Whole-Package Configuration](#whole-package-configuration)
 * [Debugging](#debugging)
 * [Testing with `barback`](#testing-with-barback)
 * [Further Reading](#further-reading)
 
 ## 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
@@ skipping 455 matching lines @@
 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.
 
+### Whole-Package Configuration
+
+For configuration that applies across multiple files, or even the entire
+package, `test` supports a configuration file called `dart_test.yaml`. At its
+simplest, this file can contain the same sort of configuration that can be
+passed as command-line arguments:
+
+```yaml
+# This package's tests are very slow. Double the default timeout.
+timeout: 2x
+
+# This is a browser-only package, so test on content shell by default.
+platforms: [content-shell]
+```
+
+The configuration file sets new defaults. These defaults can still be overridden
+by command-line arguments, just like the built-in defaults. In the example
+above, you could pass `--platform chrome` to run on Chrome instead of the
+Dartium content shell.
+
+A configuration file can do much more than just set global defaults. See
+[the full documentation][package config] for more details.
+
+[package config]: https://github.com/dart-lang/test/blob/master/doc/package_config.md
+
 ## 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
@@ skipping 66 matching lines @@
 available to tests.
 
 [api]: http://www.dartdocs.org/documentation/test/latest/index.html
 
 The test runner also supports a machine-readable JSON-based reporter. This
 reporter allows the test runner to be wrapped and its progress presented in
 custom ways (for example, in an IDE). See [the protocol documentation][json] for
 more details.
 
 [json]: https://github.com/dart-lang/test/blob/master/doc/json_reporter.md