add --ff-only flag

add --ff-only flag to cesium.com branch merge

Author: lukemckinstry

Documentation/Contributors/ReleaseGuide/README.md

@@ -73,7 +73,7 @@ There is no one release manager; instead, [our community shares the responsibili
     - `npm publish -w @cesium/engine`
     - `npm publish -w @cesium/widgets`
 29. Publish the top-level `cesium` package to npm by running `npm publish` in the repository root (not the unzipped file directory) (the first time you do this, you will need to authorize the machine using `npm adduser`).
-30. Check out the `cesium.com` branch. Merge the new release tag into the `cesium.com` branch `git merge origin <tag-name>`. CI will deploy the hosted release, Sandcastle, and the updated doc when you push the branch up.
+30. Check out the `cesium.com` branch. Merge the new release tag into the `cesium.com` branch `git merge origin <tag-name> --ff-only`. CI will deploy the hosted release, Sandcastle, and the updated doc when you push the branch up.
 31. After the `cesium.com` branch is live on cesium.com, comment in the `Communications` channel in Teams to notify comms that the release is done so they can add these highlights and publish the monthly blog post
     - Note, it may take a little while for the new version of CesiumJS to be live on cesium.com (~30 minutes after the branch builds). You can check the version of Cesium in [sandcastle](https://sandcastle.cesium.com/) by looking at the tab above the cesium pane.
 32. Continue to the [Cesium Analytics release](https://github.com/CesiumGS/cesium-analytics/tree/main/Documentation/ReleaseGuide)