Added a readme to the browser package, so people know what's up.

pkg/browser/README.md

+The dart.js file is used in Dart browser apps to check for native Dart support
+and either (a) bootstrap Dartium or (b) load compiled JS instead.  Previously,
+we've recommended that you add a script tag pointing the version of dart.js in
+our repository.  This doesn't work offline and also results in slower startup
+(see [dartbug.com/6723](http://dartbug.com/6723)).
+
+Instead, we now recommend that you install dart.js via the following steps:
+
+1. Add the following to your pubspec.yaml:
+  dependencies:
+    browser: any
+
+2. Run pub install.
+
+3. Use a relative script tag in your html to the installed version:
+
+    `<script src="packages/browser/dart.js"></script>`
+
+If you do not wish to use pub, you may host a copy of this file locally instead.
+In this case, you will need to update it yourself as necessary.  We reserve the
+right to move the old file in the repository, so we no longer recommend linking
+to it directly.