Add --wait-for-debugger to devtools' documentation

mojo/devtools/common/README.md

 # Devtools
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
 
@@ skipping 85 matching lines @@
 ```sh
 (gdb) update-symbols
 ```
 
 If you only want to update symbols for the current selected thread (for example,
 after changing threads), use the `current` option:
 ```sh
 (gdb) update-symbols current
 ```
 
+If you want to debug the startup of your application, you can pass
+`--wait-for-debugger` to `mojo_run` to have the Mojo Shell stop and wait to be
+attached by `gdb` before continuing.
+
 #### Android crash stacks
 When Mojo shell crashes on Android ("Unfortunately, Mojo shell has stopped.")
 due to a crash in native code, `mojo_debug` can be used to find and symbolize
 the stack trace present in the device log:
 
 ```sh
 mojo_debug device stack
 ```
 
 ### devtoolslib
 
 **devtoolslib** is a Python module containing the core scripting functionality
 for running Mojo apps: shell abstraction with implementations for Android and
 Linux and support for apptest frameworks. The executable scripts in devtools are
 based on this module. One can also choose to embed the functionality provided by
 **devtoolslib** in their own wrapper.
 
 ## Development
 
 The library is canonically developed [in the mojo
 repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
 https://github.com/domokit/devtools is a mirror allowing to consume it
 separately.