Document the way to debug cdt-gdb-adapter (eclipse-cdt-cloud#129)

Fixes eclipse-cdt-cloud#121 (comment)

README.md

@@ -47,4 +47,15 @@ Then from this project run
 yarn link cdt-gdb-adapter
 ```
 
-You can set up a VS Code workspace that has both folders. Also make sure you have builds running in each folder to pick up updates.
+You can set up a VS Code workspace that has both folders. Also make sure you have builds running in each folder to pick up updates (e.g. `yarn watch`).
+
+The way to debug cdt-gdb-adapter works with the same principle as the example Mock Debug Adapter provided by VSCode.
+For detailed instructions please refer to [Development Setup for Mock Debug](https://code.visualstudio.com/api/extension-guides/debugger-extension#development-setup-for-mock-debug).
+
+The short step-by-step version is:
+
+1. Launch this extension with the `Extension` launch configuration ([cdt-gdb-vscode's launch.json](https://github.com/eclipse-cdt-cloud/cdt-gdb-vscode/blob/004a59f329136c2d5eb23e11e54b1f3f51b4d197/.vscode/launch.json#L8))
+2. Launch cdt-gdb-adapter in server mode with either `Server` or `Server (Target adapter)` depending on whether you want to use local debugging or target debugging, corresponding to `"type": "gdb"` and `"type": "gdbtarget"` respectively. ([cdt-gdb-adapter's launch.json](https://github.com/eclipse-cdt-cloud/cdt-gdb-adapter/blob/92bb15046fea82256742a69f0b240129a1949a76/.vscode/launch.json#L4-L21))
+3. Add a breakpoint somewhere useful, such as [`launchRequest`](https://github.com/eclipse-cdt-cloud/cdt-gdb-adapter/blob/6ba0de8e466f4953501181f53ecdfb14c7988973/src/desktop/GDBTargetDebugSession.ts#L94)
+4. Add `"debugServer": 4711` to the launch configuration in the extension development host. The `4711` is the port number that cdt-gdb-adapter is listening on.
+5. Debug the C/C++ program in the extension development host.