//build/README.md: Add link to https://chromium.googlesource.com/

Change-Id: Idecb1fe3d962557ee30fac7bec103ca57a45edc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814880
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#698892}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bc9db99ea70706fa4ab1481bb9ae61aa12142d02

README.md

@@ -9,9 +9,11 @@ to files outside of it. Some exceptions exist (`//testing`, select
 `//third_party` subdirectories), but new dependencies tend to break these other
 projects, and so should be avoided.
 
-Changes to //build should be landed in the Chromium repo. They will then be
+Changes to `//build` should be landed in the Chromium repo. They will then be
 replicated to the stand-alone [build repo](https://chromium.googlesource.com/chromium/src/build)
 by the [gsubtreed tool.](https://chromium.googlesource.com/infra/infra/+/master/infra/services/gsubtreed)
+Note: You can find all directories already  available through gsubtreed in the
+[list of all chromium repos](https://chromium.googlesource.com/).
 
 ## Contents
  * `//build/config` - Common templates via `.gni` files.