Talk about setUp and tearDown in the README.

Closes #347

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1397903003 .

README.md

@@ -76,6 +76,35 @@ void main() {
 }
 ```
 
+You can use the [`setUp()`][setUp] and [`tearDown()`][tearDown] functions to
+share code between tests. The `setUp()` callback will run before every test in a
+group or test suite, and `tearDown()` will run after. `tearDown()` will run even
+if a test fails, to ensure that it has a chance to clean up after itself.
+
+```dart
+import "package:test/test.dart";
+
+void main() {
+  var server;
+  var url;
+  setUp(() async {
+    server = await HttpServer.bind('localhost', 0);
+    url = Uri.parse("http://${server.address.host}:${server.port}");
+  });
+
+  tearDown(() async {
+    await server.close(force: true);
+    server = null;
+    url = null;
+  });
+
+  // ...
+}
+```
+
+[setUp]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_setUp
+[tearDown]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_tearDown
+
 ## Running Tests
 
 A single test file can be run just using `pub run test:test path/to/test.dart`