Add instructions for building Release to HACKING.md

sky/HACKING.md

 Hacking on Sky
 ==============
 
 Building
 --------
 
 * Follow the setup & build instructions for [Mojo](https://github.com/domokit/mojo)
-* Build ``sky`` with ``ninja``, e.g. ``ninja -C out/Debug sky``
+* Build ``sky`` with ``ninja``, e.g. ``ninja -C out/Debug sky`` for a debug build.
+
+If you want to build release, run the following commands:
+
+* ``gn gen out/Release --args="is_debug=false"``
+* ``ninja -C out/Release sky``
 
 Running applications
 --------------------
 
 * ``./sky/tools/skydb [url]``
   * You should see a ``(skydb)`` prompt
   * Type ``help`` to see the list of available commands
+  * Note: skydb is currently hard-coded to use ``//out/Debug``
 
 * ``./sky/tools/test_sky --debug``
-  * This should run the tests
+  * This runs the tests against ``//out/Debug``. If you want to run against
+    ``//out/Release``, omit the ``--debug`` flag.
 
 Running tests manually
 ----------------------
 
 * ``sky/tools/run_sky_httpd``
 * ``out/Debug/mojo_shell --args-for="mojo://native_viewport_service/ --use-headless-config" --content-handlers=text/html,mojo://sky_viewer/ --url-mappings=mojo:window_manager=mojo:sky_tester mojo:window_manager``
 * The ``sky_tester`` should print ``#READY`` when ready
 * Type the URL you wish to run, for example ``http://127.0.0.1:8000/lowlevel/text.html``, and press the enter key
 * The harness should print the results of the test.  You can then type another URL.
 
 Writing tests
 -------------
 
 * Import ``resources/mocha.html`` and ``resources/chai.html``
 * Write tests in [mocha format](http://visionmedia.github.io/mocha/#getting-started) and use [chai asserts](http://chaijs.com/api/assert/):
 ```html
 describe('My pretty test of my subject', function() {
   var subject = new MySubject();
 
   it('should be pretty', function() {
     assert.ok(subject.isPretty);
   });
 });
 ```