Add more details to how to debug with gdb.

sky/HACKING.md

 Hacking on Sky
 ==============
 
 Building
 --------
 
 * Follow the setup & build instructions for [Mojo](https://github.com/domokit/mojo)
 
 The build directory will be `out/Debug` for Linux debug builds, and
 `out/Release` for Linux release builds. For Android builds, prefix
@@ skipping 48 matching lines @@
 We recommend using the [Dart
 ``test``](https://pub.dartlang.org/packages/test) testing framework.
 See [``sky/tests/raw/color_bounds.dart``](tests/raw/color_bounds.dart)
 for an example.
 
 Debugging Sky
 -------------
 
 This document aims to explain how to debug Sky itself.
 
-=== C++
+### C++
 
 Launch a debug Sky build on Linux as follows (where `app.dart` is the
 test you are running and trying to debug):
 
 ```bash
 mojodb start --gdb out/Debug app.dart
 mojodb gdb_attach
 ```
 
-=== Dart
+Once gdb has loaded, hit `c` to start the app. The linux simulator
+will load (slowly), after spawning many threads. When your app
+crashes, it will pause in the debugger. At that point, regular gdb
+commands will work: `n` to step over the current statement, `s` to
+step into the current statement, `f` to step out of the current block,
+`c` to continue until the next breakpoint or exception.
+
+### Dart
 
 Use Observatory.