Improve tools/run_dev.sh to allow custom flags

It is currently impossible or very hard to run tools/run_dev.sh
with a jvm debug port open.

With this change, one can simply run 'tools/run_dev.sh --debug' to open
 the default jvm debug port.

Change-Id: Ie3702affb3a7ee604cfa2dca6836135e0e8663b7

Documentation/developer-guide.md

diff --git a/Documentation/developer-guide.md b/Documentation/developer-guide.md
index 5b64e5e..d36a24d 100644
--- a/Documentation/developer-guide.md
+++ b/Documentation/developer-guide.md
@@ -70,6 +70,16 @@
 http://localhost:8080/ to view your local copy of gitiles, which
 will serve any repositories under `/path/to/repositories`.
 
+Passing `--debug` option to `tools/run_dev.sh` will suspend the runtime until a
+remote debugger connects to port 5005. If you don't want to suspend the
+runtime, make sure to assign value `n` to environment variable
+`DEFAULT_JVM_DEBUG_SUSPEND`:
+
+```
+cd /path/to/repositories  # Don't run from the gitiles repo.
+export DEFAULT_JVM_DEBUG_SUSPEND=n; /path/to/gitiles/tools/run_dev.sh --debug
+```
+
 To run unit tests, refer to the aforementioned bazel test command.
 
 ## Pushing your changes