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 48 matching lines @@
 #### Tracing
 In order to collect [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.:
 
 ```sh
 mojo_run --debugger APP_URL [--android]
 ```
 
+To trace the shell startup, use the `--trace-startup` flag:
+
+```sh
+mojo_run --trace-startup APP_URL [--android]

[ppi, 2015/09/07 08:56:29]

Actually, this should not be placed under ### Debugger. Debugger offers
interactive tracing, while this is shell feature. Can we put this under ###
Runner?

[etiennej, 2015/09/07 09:05:49]

Yet, this is related to tracing, and this sub-section is called "Tracing". Note
that even the "interactive tracing" needs you to run mojo_run with a
tracing-specific argument (by reading the documentation, one would think that
the --debugger flag is only used for tracing).

Do we really want two "tracing" subsections? Shouldn't we have a "### Tracing"
section at the same level than Debugger and Runner?

[etiennej, 2015/09/07 10:41:51]

Reworded per offline discussion.

+```
+
 Then, in another shell, tracing can be orchestrated as follows:
 
 ```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
 `about://tracing`.
 
@@ skipping 35 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.