Add missing information about updating changelog (#399)

DEVELOPER.md

@@ -2,8 +2,13 @@
 
 ## Publish
 
+Before publishing, please make sure CHANGELOG.md is up-to-date, because this
+will be displayed in the [changelog tab][] in the Marketplace. Please only add
+changes directly relevant to users there, e.g., no changes about tests and CI.
+
 To publish a newer version to the Market Place, follow the [VSCode official
 instructions][]. Then,
 [Open VSX should update automatically](https://github.com/EclipseFdn/open-vsx.org/wiki/Auto-Publishing-Extensions#how-to-auto-publish-an-extension).
 
 [VSCode official instructions]: https://code.visualstudio.com/api/working-with-extensions/publishing-extension
+[changelog tab]: https://marketplace.visualstudio.com/items/EditorConfig.EditorConfig/changelog