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

R=ppi@chromium.org, ppi
BUG=

Review URL: https://codereview.chromium.org/1257873005 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: b6b07c7025cab306b3f05f3e0601a2488409f3a9

README.md

diff --git a/README.md b/README.md
index 4536ea5..5022548 100644
--- a/README.md
+++ b/README.md
@@ -103,6 +103,10 @@
 (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

mojo_run

diff --git a/mojo_run b/mojo_run
index 0adba39..ee43274 100755
--- a/mojo_run
+++ b/mojo_run
@@ -16,7 +16,8 @@
          "[--enable-external-applications] "
          "[--disable-cache] "
          "[--enable-multiprocess] "
-         "[<mojo-app>] "
+         "[--wait-for-debugger] "
+         "[--sky <mojo-app>|<mojo-app>] "
          """
 
 A <mojo-app> is a Mojo URL or a Mojo URL and arguments within quotes.