Add asset licensing notices (#11397)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: surfdude29 <149612116+surfdude29@users.noreply.github.com>

assets/icons/README.md

@@ -0,0 +1,23 @@
+The user-interface glyphs in this directory are from the
+[Central icon system](https://iconists.co/central) by Iconists (David & Storm GbR). Bluesky
+Social PBC licenses them for use in our own products. **That license is for our own use, and it
+does not extend to you.**
+
+**These icons are not covered by the [MIT license](../../LICENSE) that applies to the rest of
+this repository.** If you want to use them, buy a license from
+[iconists.co](https://iconists.co). There are also good openly licensed icon sets if you would
+rather not.
+
+This notice covers the SVG files at the top level of this directory. It does not cover:
+
+- `flags/` — country-flag-icons, MIT licensed, see [`flags/README.md`](./flags/README.md)
+- `community/` — third-party service marks, see [`community/README.md`](./community/README.md)
+- `logomark.svg`, `newskie.svg`, `verifiedCheck.svg`, `verifierCheck.svg`, `starterPack.svg`,
+  `starter_pack_icon.svg` — Bluesky trademarks and brand assets
+- `custom_logo_japan.svg` — artwork by a Bluesky Japan logo contest entrant
+- `apple_logo.svg`, `android_logo.svg` — Apple and Google trademarks
+
+Each of those is described in [`ASSETS.md`](../../ASSETS.md).
+
+Adding an icon here? If it came from Central, this notice already covers it. If it came from
+anywhere else, add it to `ASSETS.md` so the notice does not go stale.