docs: update release instructions for Github releases (#10192)

RELEASING/README.md

@@ -308,6 +308,13 @@ Once it's all done, an [ANNOUNCE] thread announcing the release to the dev@ mail
     (venv)$ python send_email.py announce
 ```
 
+### Github Release
+
+Finally, so the Github UI reflects the latest release, you should create a release from the
+tag corresponding with the new version. Go to https://github.com/apache/incubator-superset/tags,
+click the 3-dot icon and select `Create Release`, paste the content of the ANNOUNCE thread in the
+release notes, and publish the new release.
+
 ## Post release
 
 #### Refresh documentation website