clarify how to turn off syncExtensions for release

Author: akosyakov

docs/releasing.md

@@ -4,6 +4,7 @@
     - Update version of the extension - this is usually the minor version. **Until the marketplace supports semantic versioning, the minor version should always be an even number. Odd numbers are reserved for the pre-release version of the extension.**
     - Turn off experimental settings:
       - `gitpod.remote.useLocalApp: true`
+      - `gitpod.remote.syncExtensions: false`
     - (If necessary) Update vscode engine version
 
 2. If the minor version was increased, run the Nightly action to ensure a new pre-release version with the increased version number is released