Update mapstore-migration-guide.md

Updated manually on the branch. The PR #6859  didn't contain this update because of a conflict not committed in error.

docs/developer-guide/mapstore-migration-guide.md

@@ -20,6 +20,12 @@ This is a list of things to check if you want to update from a previous version
 - Optionally check also accessory files like `.eslinrc`, if you want to keep aligned with lint standards.
 - Follow the instructions below, in order, from your version to the one you want to update to.
 
+## Migration from 2021.01.01 to 2021.01.03
+
+Generally speaking this is not properly a breaking change, but more a fix to apply to your installations. Certificate for 'cesiumjs.org' has expired at 2021.05.05, so to continue using CesiumJS with MapStore
+you will have to replace all the URLs like `https://cesiumjs.org/releases/1.17` in `https://cesium.com/downloads/cesiumjs/releases/1.17`. This is the main fix of this minor release.
+See [this pull request on GitHub](https://github.com/geosolutions-it/MapStore2/pull/6856) as a sample to apply these changes to your project.
+
 ## Migration from 2021.01.00 to 2021.01.01
 
 ### Update embedded entry to load the correct configuration