Mention the test/pub_serve transformer in the README

Author: greglittlefield-wf

README.md

@@ -306,7 +306,7 @@ configuration from the `config.test` object.
             <td><code>pubServe</code></td>
             <td><code>bool</code></td>
             <td><code>false</code></td>
-            <td>Whether or not to serve browser tests using a Pub server.</td>
+            <td>Whether or not to serve browser tests using a Pub server.<br>If <code>true</code>, make sure to follow the <code>test</code> package's <a href="https://github.com/dart-lang/test#testing-with-barback">setup instructions</a> and include the <code>test/pub_serve</code> transformer.</td>
         </tr>
     </tbody>
 </table>
@@ -423,7 +423,7 @@ object.
             <td><code>pubServe</code></td>
             <td><code>bool</code></td>
             <td><code>false</code></td>
-            <td>Whether or not to serve browser tests using a Pub server.</td>
+            <td>Whether or not to serve browser tests using a Pub server.<br>If <code>true</code>, make sure to follow the <code>test</code> package's <a href="https://github.com/dart-lang/test#testing-with-barback">setup instructions</a> and include the <code>test/pub_serve</code> transformer.</td>
         </tr>
     </tbody>
 </table>