Clarify that announcements section doesn't specify every terriajs version upgrade

TerriaMap users may expect every terriajs version upgrade to be announced here. This attempts to clarify that only major upgrades that may require significant work for a TerriaMap fork maintainer will be announced here

Author: tephenavies

README.md

@@ -15,6 +15,10 @@ To get in touch:
 
 ---
 
+## Major announcements
+
+Following is a list of major announcements and upgrades that may affect users maintaining a fork (copied from [TerriaJS announcements](https://github.com/TerriaJS/terriajs/discussions/categories/announcements)). For a full list of changes to TerriaMap, including the latest versions of TerriaJS included with each release please refer to [CHANGES.md](https://github.com/TerriaJS/TerriaMap/blob/main/CHANGES.md). 
+
 ### We have released TerriaJS v8.3.0 (2023-05-22)
 
 Terriajs version `8.3.0` includes a few breaking changes: