docs: add a rewritten trybot usage doc.

Tbr: martiniss@chromium.org
Bug: 904636
Change-Id: I7e8d7ed29da753a1813f304d17efccb8a327f441
Reviewed-on: https://chromium-review.googlesource.com/c/1347612Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610257}

docs/infra/trybot_usage.md

+# Using the trybots
+
+[TOC]
+
+## Overview
+
+The trybots let committers try uncommitted patches on multiple platforms in
+an automated way.
+
+ - Trybots include all platforms for which we currently build Chromium, though
+   they may not support all configurations built on CI.
+ - The commit queue runs a subset of available trybots. See [here][1] for more
+   information.
+ - trybots can be manually invoked via `git cl try` or the "Choose Trybots"
+   UI in gerrit.
+ - Any committer can use the trybots.
+ - Non-committers with tryjob access can also use the trybots. See [here][2]
+   for more information.
+ - External contributors without tryjob access can ask committers to run
+   tryjobs for them.
+
+## Workflow
+
+1. Upload your change to gerrit via `git cl upload`
+2. Run trybots:
+
+    * Run the default set of trybots by starting a CQ dry run, either by
+      setting CQ+1 on gerrit or by running `git cl try` with no arguments.
+    * Run trybots of your choice by providing arguments to `git cl try`:
+
+        * specify bucket name with `-B/--bucket`. For chromium tryjobs, this
+          should always be `luci.chromium.try`
+        * specify bot names with `-b/--bot`. This can be specified more than once.
+
+### Examples
+
+Launching a CQ dry run:
+
+```bash
+$ git cl try
+```
+
+Launching a particular trybot:
+
+```bash
+$ git cl try -B luci.chromium.try -b linux_chromium_rel_ng
+```
+
+Launching multiple trybots:
+
+```bash
+$ git cl try -B luci.chromium.try \
+  -b android-binary-size \
+  -b ios-simulator-full-configs \
+  -b linux-blink-rel \
+  -b win7-blink-rel
+  # etc
+```
+
+## Bugs? Feature requests? Questions?
+
+[File a trooper bug.][3]
+
+## Legacy documentation
+
+ - [Design doc][4]
+
+[1]: /docs/infra/cq.md
+[2]: https://www.chromium.org/getting-involved/become-a-committer#TOC-Try-job-access
+[3]: https://g.co/bugatrooper
+[4]: https://www.chromium.org/developers/testing/try-server-usage/design