Document the --trace-startup shell flag.

R=ppi@chromium.org, ppi

Review URL: https://codereview.chromium.org/1329963002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 94c26b19d3d2580bd41b516b08ddbb1079dfa02b

README.md

diff --git a/README.md b/README.md
index e6044b2..98cfaa4 100644
--- a/README.md
+++ b/README.md
@@ -57,23 +57,33 @@
 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