docs: Mention the CIPD GN template in cipd.md.

Change-Id: Ica186f98a2a620281447db4965ad2788e7dba85f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2181427
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766916}

docs/cipd.md

@@ -62,7 +62,33 @@ lay them out like so:
 ### 3. Create a cipd.yaml file
 
 CIPD knows how to create your package based on a .yaml file you provide to it.
-The .yaml file should take the following form:
+This file can either be generated by a GN template, or manually.
+
+#### 3a. Generating cipd.yaml via GN Template:
+
+The `cipd_package_definition` template in [build/cipd/cipd.gni] can be used to
+create the yaml definition as part of Chromium's normal build process. Declare
+a target like:
+```
+cipd_package_definition("my_cipd_package") {
+  package = "path/to/cipd/package"
+  description = "Prebuilt test binary."
+  install_mode = "copy"
+  deps = [ "//path/to:test_binary_target" ]
+  sources = [ "//path/to:test_binary_file" ]
+}
+```
+
+After generating build files, you can then run
+`ninja -C out/Default my_cipd_package`, which creates the .yaml file under
+`out/Default/my_cipd_package/package.yaml`.
+
+[build/cipd/cipd.gni]: https://source.chromium.org/chromium/chromium/src/+/master:build/cipd/cipd.gni
+
+#### 3b. Generating cipd.yaml by hand:
+
+You can also write the .yaml file by hand. The file should take the following
+form:
 
 ```
 # Comments are allowed.
@@ -122,17 +148,13 @@ data:
   - file: foo.jar
 ```
 
-To create a private (Googler-only) package:
-```
-# Map this to //clank/third_party/sample_cipd_dep.
-package: chrome_internal/third_party/sample_cipd_dep
-```
-
 For more information about the package definition spec, see [the code].
 
-> **Note:** Committing the .yaml file to the repository isn't required,
-> but it is recommended. Doing so has benefits for visibility and ease of
-> future updates.
+> **Note:** To make the package private (Googler-only), prefix the package name
+> with `chrome_internal`. For example:
+> ```
+> package: chrome_internal/third_party/sample_cipd_dep
+> ```
 
 [the code]: https://chromium.googlesource.com/infra/luci/luci-go/+/master/cipd/client/cipd/builder/pkgdef.go