contributor docs: Document how to add and modify icons.

docs/subsystems/icons.md

@@ -0,0 +1,28 @@
+# Icons
+
+## Adding a new icon
+
+A new feature may require a new icon to represent it, for example to be used
+next to a menu option. The issue you're working on may not have an icon
+specified upfront. In this case, you should:
+
+1. Prototype using a [Lucide icon](https://lucide.dev/icons/), which is the
+   default source for icons in Zulip.
+
+1. **When your feature is nearing completion**, post in the [appropriate
+   channel](https://zulip.com/development-community/#where-do-i-send-my-message)
+   in the Zulip development community to discuss what icon should be used (e.g.,
+   #design for web app icons). You can use the discussion thread linked from the
+   issue you're working on if there is one.
+
+1. Once there is general consensus, @-mention Zulip's designer (Vlad Korobov) to
+   ask him to prepare the icon to be used.
+
+1. Follow the [attribution guidelines](../contributing/licensing.md)
+   to document the icon source.
+
+## Changing an icon
+
+When changing an icon for an existing feature, be sure to [update the help
+center](../documentation/helpcenter.md#icons) accordingly (`git grep` is your
+friend).

docs/subsystems/index.md

@@ -8,6 +8,7 @@ maxdepth: 3
 dependencies
 settings
 html-css
+icons
 accessibility
 events-system
 sending-messages