Document the --trace-startup shell flag.

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 39 matching lines @@
 By default, `mojo_run` uses https://core.mojoapps.io/kiosk_wm.mojo as the window
 manager. You can pass a different window manager url using the
 `--window-manager` flag to override this.
 
 ### Debugger
 
 `mojo_debug` allows you to interactively inspect a running shell, collect
 performance traces and attach a gdb debugger.
 
 #### Tracing
-In order to collect [performance
+[Performance
 traces](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool)
-interactively through `mojo_debug`, make sure that the app being inspected was
-run with `--debugger` switch. E.g.:
+can either be collected by Mojo Shell during its startup, or collected
+interactively by `mojo_debug`.
+
+To trace the Mojo Shell startup, use the `--trace-startup` flag:
+
+```sh
+mojo_run --trace-startup APP_URL [--android]
+```
+
+In order to collect traces interactively through `mojo_debug`, make sure that
+the app being inspected was run with `--debugger` switch. E.g.:
 
 ```sh
 mojo_run --debugger APP_URL [--android]
 ```
 
-Then, in another shell, tracing can be orchestrated as follows:
+While Mojo Shell is running, tracing can be started and stopped by these two
+commands respectively:
 
 ```sh
 mojo_debug tracing start
 mojo_debug tracing stop [result.json]
 ```
 
-The trace file can be then loaded using the trace viewer in Chrome available at
+Trace files can be then loaded using the trace viewer in Chrome available at
 `about://tracing`.
 
 #### GDB
 It is possible to inspect a Mojo Shell process using GDB. The `mojo_debug`
 script can be used to launch GDB and attach it to a running shell process
 (android only):
 
 ```sh
 mojo_debug gdb attach
 ```
@@ skipping 27 matching lines @@
 ```sh
 mojo_debug device stack
 ```
 
 ## 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.