Add step for release assets (Leaflet#8494)

Co-authored-by: Florian Bischof <[email protected]>
Co-authored-by: Jon Koops <[email protected]>

RELEASE.md

@@ -9,6 +9,7 @@
   - [ ] [Leaflet NPM package page](https://www.npmjs.com/package/leaflet)
   - [ ] files on [Leaflet unpkg page](https://unpkg.com/leaflet@latest/)
 - [ ] Make a new release on [Leaflet's GitHub release page](https://github.com/Leaflet/Leaflet/releases/) with the most important parts of the changelog
+- [ ] Download zip archive from https://leafletjs-cdn.s3.amazonaws.com/content/leaflet/vX.X.X/leaflet.zip (where `X.X.X` is the new version number) and upload it as an "asset" of the GitHub release.
 
 ### Updating docs after the release