Prepare changelog for v1.8.3-rc.2

Author: RiotRobot

CHANGELOG.md

@@ -1,3 +1,9 @@
+Changes in [1.8.3-rc.2](https://github.com/vector-im/element-desktop/releases/tag/v1.8.3-rc.2) (2021-09-09)
+===========================================================================================================
+
+## 🐛 Bug Fixes
+ * Debounce read marker update on scroll ([\#6774](https://github.com/matrix-org/matrix-react-sdk/pull/6774)).
+
 Changes in [1.8.3-rc.1](https://github.com/vector-im/element-desktop/releases/tag/v1.8.3-rc.1) (2021-09-07)
 ===========================================================================================================