Mention how to convert "copies" from gyp to gn in the cookbook.

BUG=none

Review URL: https://codereview.chromium.org/1683123002

Cr-Original-Commit-Position: refs/heads/master@{#374791}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: bf781415f643cacdb62968ef765f83fa12674ac6

tools/gn/docs/cookbook.md

diff --git a/tools/gn/docs/cookbook.md b/tools/gn/docs/cookbook.md
index c918d86..c328563 100644
--- a/tools/gn/docs/cookbook.md
+++ b/tools/gn/docs/cookbook.md
@@ -64,7 +64,7 @@
   args = [ "--la_dee_da" ]
 }
 
-executable('foo.exe') {
+executable("foo.exe") {
   ...
   deps = [ ":foo" ]  # Depend on the action to make sure it runs.
 }
@@ -73,6 +73,36 @@
 Rules in GYP become `action_foreach` in GN which work like actions but
 iterate over a set of sources.
 
+### Copies
+
+GYP
+
+```
+'copies': [
+  {
+    'destination': '<(PRODUCT_DIR)/',
+    'files': [
+      '../build/win/dbghelp_xp/dbghelp.dll',
+    ],
+  },
+],
+```
+
+Unlike GYP, where copies are part of a target, GN copies are
+separate targets that you then depend on via deps from other targets:
+
+```
+copy("bar") {
+  sources = [ "../path/to/secret.dll" ]
+  outputs = [ "$root_out_dir/{{source_file_part}}" ]
+}
+
+component("base") {
+  ...
+  deps = [ "bar" }  # Depend on the copy to make sure it runs.
+}
+```
+
 ## Platform checking
 
 | *GYP*                                | *GN*                 |