Add basic support for a configuration file.

README.md

Index: README.md
diff --git a/README.md b/README.md
index beda9ef1825c43803c57580ae7c67fca446211e6..cabbd1f7915a14dd638eadd9842fb380b5845192 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@
   * [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)
@@ -486,6 +487,31 @@ 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,