Add a doc outlining how to use led to debug/change a chromium builder.

I've received enough questions regarding led & chromium bots to warrant
a small doc, I think. This isn't meant to be a full overview of what's
possible with led, but might be a good starting point for someone that's
not familiar with it. (ie: The target audience is someone that's not very
familiar with our infra/bots.)

Change-Id: I629f3872aa5f0b2da0ec4821ba1befea47bdd3da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518458Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708660}

docs/infra/using_led.md

+# Using LED
+
+LED is an infrastructure tool used to manually trigger builds on any builder
+running on LUCI. It's designed to help debug build failures or experiment with
+new builder changes. This doc describes how to use it with Chromium's builders.
+
+[TOC]
+
+## When to use it
+
+Use cases include, but are not limited to, the following:
+* **Testing a recipe change**: Much of the code in the following repos
+define what and how a builder runs (also known as the builder's "recipe"):
+[recipes-py][1], [depot_tools][2], and [tools/build][3]. Changes to these
+repos can first be tested out on any builder prior to submitting via LED.
+* **Debugging a waterfall failure**: If a waterfall builder (that is, *not* a
+trybot) is exhibitting frequent or strange failures that can't be reproduced
+locally, LED can be used to retrigger any given build for debugging.
+
+## When *not* to use it
+
+Certain types of changes to a trybot (this includes all builders on the CQ)
+can be sufficiently tested without the use of LED. This includes changes to a
+trybot's:
+* **GN args**: A trybot's build args are configured via
+[mb_config.pyl][4].
+* **tests**: The list of tests a trybot runs are set via the \*.pyl files in
+[//testing/buildbot/][5]. (Some trybots may not be present in
+those files. Instead, change the waterfall builders they mirror. This mapping is
+configured in tools/build's [trybots.py][6].)
+
+Simply edit the needed files in a local chromium/src checkout, upload the change
+to Gerrit, then select the affected trybot(s) via the "select tryjobs" menu.
+
+## How to use it
+
+Provided that a local depot_tools checkout is present on $PATH, LED can be
+used by simply invoking `led` on the command line. A common use-case for LED is
+to modify the build steps of any given builder. The process for doing this is
+outlined below. (However, LED can be used for many other purposes. See the full
+list of features via `led help`.)
+
+1. Select a builder whose builds you'd like to reproduce. (Example:
+[linux-rel][7])
+2. Record its full builder name, along with its bucket. (The bucket name is
+present in the URL of the builder page, and is very likely "chromium/ci".)
+3. Checkout the [tools/build][3] repo (if not already present) and navigate to
+the [chromium][8] and/or [chromium_tests][9] recipe modules. These, along with
+the other recipe_modules located in tools/build, are how the majority of a
+Chromium builder's recipe is defined.
+4. Make the desired recipe change. (Consider running local recipe unittests
+before proceeding by running `recipes.py test train` via the [recipes.py][10]
+script.
+5. Launch a build with the given recipe change. This can be done with a single
+chained LED invocation, eg:
+`led get-builder chromium/ci:linux-rel | led edit-recipe-bundle | led launch`
+6. The LED invocation above will print out a link to the build that was
+launched. Repeat steps 4 & 5 until the triggered builds behave as expected
+with the new recipe change.
+
+## Questions? Feedback?
+
+If you're in need of further assistance, if you're not sure about
+one or more steps, or if you found this documentation lacking, please
+reach out to infra-dev@chromium.org or [file a bug][11]!
+
+[1]: https://chromium.googlesource.com/infra/luci/recipes-py/
+[2]: https://chromium.googlesource.com/chromium/tools/depot_tools/
+[3]: https://chromium.googlesource.com/chromium/tools/build/
+[4]: /tools/mb/mb_config.pyl
+[5]: /testing/buildbot/
+[6]: https://chromium.googlesource.com/chromium/tools/build/+/HEAD/scripts/slave/recipe_modules/chromium_tests/trybots.py
+[7]: https://ci.chromium.org/p/chromium/builders/ci/linux-rel
+[8]: https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave/recipe_modules/chromium/api.py
+[9]: https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave/recipe_modules/chromium_tests/api.py
+[10]: https://chromium.googlesource.com/chromium/tools/build/+/master/scripts/slave/recipes.py
+[11]: https://g.co/bugatrooper