Update the documentation for //infra/config.

Change-Id: I33f82f7b93f9815c625cf6223492d2ebc66ea254
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134193Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756322}

infra/config/README.md

@@ -3,25 +3,50 @@
 
 This directory contains chromium project-wide configurations
 for Chrome Operations services.
-For example, [cr-buildbucket.cfg](cr-buildbucket.cfg) defines builders.
+For example, [cr-buildbucket.cfg](generated/cr-buildbucket.cfg) defines
+builders.
 
 **Remember** Change these configs on `master` branch only!
 
 Currently active version can be checked at
 https://luci-config.appspot.com/#/projects/chromium .
 
-The configuration files are currently in the process of being migrated to
-lucicfg/starlark. See
+The configuration is written using starlark, which is executed using lucicfg to
+generate the raw cfg files located in [generated](generated). See
 https://chromium.googlesource.com/infra/luci/luci-go/+/HEAD/lucicfg/doc/README.md
-for more information on lucicfg/starlark. If a hand-written configuration file
-is still present alongside this file, you can modify that file directly.
-
-The remainder of the configuration files are generated by starlark. The starlark
-configuration is rooted in main.star and dev.star, which execute other starlark
-files to generate a subset of the LUCI service configuration files to the
-`generated` subdirectory. A presubmit check enforces that the generated files
-are kept in sync with the generated output of the starlark configuration.
-
-The starlark configuration also copies over the not-yet migrated files to the
-`generated` directory, so updating the hand-written configuration files will
-require re-'generating' the configuration.
+for more information on lucicfg/starlark.
+
+The starlark configuration is rooted in `main.star` and `dev.star`, which
+execute other starlark files to generate a subset of the LUCI service
+configuration files. A presubmit check enforces that the generated files are
+kept in sync with the output of the starlark configuration.
+
+The configuration rooted at [main.star](main.star) defines the LUCI services
+configuration for the chromium project on the production instance of LUCI. The
+configuration is responsible for generating the raw configuration files that do
+not end in -dev.cfg as well as the markdown file
+[cq-builders.md](generated/cq-builders.md). Starlark files in the following
+directories are consumed by the configuration:
+
+*   buckets - Definitions of builders for the chromium project. There is a .star
+    file for each bucket in the chromium project where the bucket and the
+    builders for that bucket are defined.
+*   consoles - Definitions of milo consoles for the chromium project. There is a
+    .star for each console that defines the console.
+*   generators - Definitions of lucicfg generators that do various things to
+    post-process the LUCI configuration before the output files are generated
+    (e.g. generate no-op jobs to workaround limitations of our recipe config) or
+    generate additional files (e.g. the CQ builders markdown document).
+*   validators - Definitions of lucicfg generators that perform additional
+    validation on the the LUCI configuration (e.g. ensure all builders are added
+    to at least one console).
+*   versioned - Definitions of builders for the main waterfall for the chromium
+    project. Builders on the main waterfall are branched so that when a new
+    milestone is cut, ci and try buckets specific to that milestone are created
+    with an equivalent set of builders, schedulers, CQ groups, etc. to enable
+    CI/CQ on the new branch.
+
+The configuration rooted at [dev.star](dev.star) defines the LUCI services
+configuration for the chromium project on the dev instance of LUCI. This
+configuration is responsible for generating the raw configuration files ending
+in -dev.cfg.