Document the --trace-startup shell flag.

mojo/devtools/common/README.md

Index: mojo/devtools/common/README.md
diff --git a/mojo/devtools/common/README.md b/mojo/devtools/common/README.md
index e6044b238e962bbba77b11bb61f55a396413d47c..ef3c785e4a4c2d36081038ddbff2fe4d97ba983b 100644
--- a/mojo/devtools/common/README.md
+++ b/mojo/devtools/common/README.md
@@ -66,6 +66,12 @@ run with `--debugger` switch. E.g.:
 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