Add note about trademarked resources to vector_icons/README.md

Bug: none
Change-Id: I5cdf1b9bcd9ae0657ed93c374579c07d99ffc4a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001844Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732056}

components/vector_icons/README.md

@@ -28,6 +28,8 @@ Some SVGs are already pretty minimal, like the ones at [the Material Design Icon
 
 Once you have created an `.icon` file, place it in an appropriate `vector_icon` subdirectory and add the filename to the corresponding `BUILD.gn`. A constant is automatically generated so that the icon can be referenced at runtime. The icon file `foo_bar.icon` is mapped to the constant name of `kFooBarIcon` ('k' + camel-cased filename + 'Icon'), which you can use to reference that icon in code. The icon's name should match its identifier on [the MD icons site](https://material.io/icons/) if that's where it came from. For example, `ic_accessibility` would become `accessibility.icon`.
 
+Make sure not to add [trademarked resources](../../docs/google_chrome_branded_builds.md) such as Google product logos to the Chromium repo.
+
 ### Icons with multiple definitions
 
 To add multiple icon definitions to a single `.icon` file, place the definitions generated by Skiafy in descending order of size. Each definition after the first must start with a `CANVAS_DIMENSIONS` directive.